Merge branch 'tb/multi-pack-verbatim-reuse'

Streaming spans of packfile data used to be done only from a single, primary, pack in a repository with multiple packfiles. It has been extended to allow reuse from other packfiles, too. * tb/multi-pack-verbatim-reuse: (26 commits) t/perf: add performance tests for multi-pack reuse pack-bitmap: enable reuse from all bitmapped packs pack-objects: allow setting `pack.allowPackReuse` to "single" t/test-lib-functions.sh: implement `test_trace2_data` helper pack-objects: add tracing for various packfile metrics pack-bitmap: prepare to mark objects from multiple packs for reuse pack-revindex: implement `midx_pair_to_pack_pos()` pack-revindex: factor out `midx_key_to_pack_pos()` helper midx: implement `midx_preferred_pack()` git-compat-util.h: implement checked size_t to uint32_t conversion pack-objects: include number of packs reused in output pack-objects: prepare `write_reused_pack_verbatim()` for multi-pack reuse pack-objects: prepare `write_reused_pack()` for multi-pack reuse pack-objects: pass `bitmapped_pack`'s to pack-reuse functions pack-objects: keep track of `pack_start` for each reuse pack pack-objects: parameterize pack-reuse routines over a single pack pack-bitmap: return multiple packs via `reuse_partial_packfile_from_bitmap()` pack-bitmap: simplify `reuse_partial_packfile_from_bitmap()` signature ewah: implement `bitmap_is_empty()` pack-bitmap: pass `bitmapped_pack` struct to pack-reuse functions ...
2025-01-23 07:53:37 +08:00 · 2024-01-12 16:09:56 -08:00 · 2024-01-12 16:09:56 -08:00 · 0fea6b73f1
commit 0fea6b73f1
parent 0ebbaa07d0 ba47d88795
21 changed files with 1036 additions and 195 deletions
--- a/Documentation/config/pack.txt
+++ b/Documentation/config/pack.txt
@ -28,11 +28,17 @@ all existing objects. You can force recompression by passing the -F option
 to linkgit:git-repack[1].

 pack.allowPackReuse::
-	When true, and when reachability bitmaps are enabled,
-	pack-objects will try to send parts of the bitmapped packfile
-	verbatim. This can reduce memory and CPU usage to serve fetches,
-	but might result in sending a slightly larger pack. Defaults to
-	true.
+	When true or "single", and when reachability bitmaps are
+	enabled, pack-objects will try to send parts of the bitmapped
+	packfile verbatim. When "multi", and when a multi-pack
+	reachability bitmap is available, pack-objects will try to send
+	parts of all packs in the MIDX.
+
+	If only a single pack bitmap is available, and
+	`pack.allowPackReuse` is set to "multi", reuse parts of just the
+	bitmapped packfile. This can reduce memory and CPU usage to
+	serve fetches, but might result in sending a slightly larger
+	pack. Defaults to true.

 pack.island::
 	An extended regular expression configuring a set of delta
--- a/Documentation/gitformat-pack.txt
+++ b/Documentation/gitformat-pack.txt
@ -396,6 +396,15 @@ CHUNK DATA:
 	    is padded at the end with between 0 and 3 NUL bytes to make the
 	    chunk size a multiple of 4 bytes.

+	Bitmapped Packfiles (ID: {'B', 'T', 'M', 'P'})
+	    Stores a table of two 4-byte unsigned integers in network order.
+	    Each table entry corresponds to a single pack (in the order that
+	    they appear above in the `PNAM` chunk). The values for each table
+	    entry are as follows:
+	    - The first bit position (in pseudo-pack order, see below) to
+	      contain an object from that pack.
+	    - The number of bits whose objects are selected from that pack.
+
 	OID Fanout (ID: {'O', 'I', 'D', 'F'})
 	    The ith entry, F[i], stores the number of OIDs with first
 	    byte at most i. Thus F[255] stores the total
@ -509,6 +518,73 @@ packs arranged in MIDX order (with the preferred pack coming first).
 The MIDX's reverse index is stored in the optional 'RIDX' chunk within
 the MIDX itself.

+=== `BTMP` chunk
+
+The Bitmapped Packfiles (`BTMP`) chunk encodes additional information
+about the objects in the multi-pack index's reachability bitmap. Recall
+that objects from the MIDX are arranged in "pseudo-pack" order (see
+above) for reachability bitmaps.
+
+From the example above, suppose we have packs "a", "b", and "c", with
+10, 15, and 20 objects, respectively. In pseudo-pack order, those would
+be arranged as follows:
+
+    |a,0|a,1|...|a,9|b,0|b,1|...|b,14|c,0|c,1|...|c,19|
+
+When working with single-pack bitmaps (or, equivalently, multi-pack
+reachability bitmaps with a preferred pack), linkgit:git-pack-objects[1]
+performs ``verbatim'' reuse, attempting to reuse chunks of the bitmapped
+or preferred packfile instead of adding objects to the packing list.
+
+When a chunk of bytes is reused from an existing pack, any objects
+contained therein do not need to be added to the packing list, saving
+memory and CPU time. But a chunk from an existing packfile can only be
+reused when the following conditions are met:
+
+  - The chunk contains only objects which were requested by the caller
+    (i.e. does not contain any objects which the caller didn't ask for
+    explicitly or implicitly).
+
+  - All objects stored in non-thin packs as offset- or reference-deltas
+    also include their base object in the resulting pack.
+
+The `BTMP` chunk encodes the necessary information in order to implement
+multi-pack reuse over a set of packfiles as described above.
+Specifically, the `BTMP` chunk encodes three pieces of information (all
+32-bit unsigned integers in network byte-order) for each packfile `p`
+that is stored in the MIDX, as follows:
+
+`bitmap_pos`:: The first bit position (in pseudo-pack order) in the
+  multi-pack index's reachability bitmap occupied by an object from `p`.
+
+`bitmap_nr`:: The number of bit positions (including the one at
+  `bitmap_pos`) that encode objects from that pack `p`.
+
+For example, the `BTMP` chunk corresponding to the above example (with
+packs ``a'', ``b'', and ``c'') would look like:
+
+[cols="1,2,2"]
+|===
+| |`bitmap_pos` |`bitmap_nr`
+
+|packfile ``a''
+|`0`
+|`10`
+
+|packfile ``b''
+|`10`
+|`15`
+
+|packfile ``c''
+|`25`
+|`20`
+|===
+
+With this information in place, we can treat each packfile as
+individually reusable in the same fashion as verbatim pack reuse is
+performed on individual packs prior to the implementation of the `BTMP`
+chunk.
+
 == cruft packs

 The cruft packs feature offer an alternative to Git's traditional mechanism of
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@ -218,13 +218,19 @@ static int thin;
 static int num_preferred_base;
 static struct progress *progress_state;

-static struct packed_git *reuse_packfile;
+static struct bitmapped_pack *reuse_packfiles;
+static size_t reuse_packfiles_nr;
+static size_t reuse_packfiles_used_nr;
 static uint32_t reuse_packfile_objects;
 static struct bitmap *reuse_packfile_bitmap;

 static int use_bitmap_index_default = 1;
 static int use_bitmap_index = -1;
-static int allow_pack_reuse = 1;
+static enum {
+	NO_PACK_REUSE = 0,
+	SINGLE_PACK_REUSE,
+	MULTI_PACK_REUSE,
+} allow_pack_reuse = SINGLE_PACK_REUSE;
 static enum {
 	WRITE_BITMAP_FALSE = 0,
 	WRITE_BITMAP_QUIET,
@ -1010,7 +1016,9 @@ static off_t find_reused_offset(off_t where)
 	return reused_chunks[lo-1].difference;
 }

-static void write_reused_pack_one(size_t pos, struct hashfile *out,
+static void write_reused_pack_one(struct packed_git *reuse_packfile,
+				  size_t pos, struct hashfile *out,
+				  off_t pack_start,
 				  struct pack_window **w_curs)
 {
 	off_t offset, next, cur;
@ -1020,7 +1028,8 @@ static void write_reused_pack_one(size_t pos, struct hashfile *out,
 	offset = pack_pos_to_offset(reuse_packfile, pos);
 	next = pack_pos_to_offset(reuse_packfile, pos + 1);

-	record_reused_object(offset, offset - hashfile_total(out));
+	record_reused_object(offset,
+			     offset - (hashfile_total(out) - pack_start));

 	cur = offset;
 	type = unpack_object_header(reuse_packfile, w_curs, &cur, &size);
@ -1088,41 +1097,93 @@ static void write_reused_pack_one(size_t pos, struct hashfile *out,
 	copy_pack_data(out, reuse_packfile, w_curs, offset, next - offset);
 }

-static size_t write_reused_pack_verbatim(struct hashfile *out,
+static size_t write_reused_pack_verbatim(struct bitmapped_pack *reuse_packfile,
+					 struct hashfile *out,
+					 off_t pack_start,
 					 struct pack_window **w_curs)
 {
-	size_t pos = 0;
+	size_t pos = reuse_packfile->bitmap_pos;
+	size_t end;

-	while (pos < reuse_packfile_bitmap->word_alloc &&
-			reuse_packfile_bitmap->words[pos] == (eword_t)~0)
-		pos++;
+	if (pos % BITS_IN_EWORD) {
+		size_t word_pos = (pos / BITS_IN_EWORD);
+		size_t offset = pos % BITS_IN_EWORD;
+		size_t last;
+		eword_t word = reuse_packfile_bitmap->words[word_pos];

-	if (pos) {
-		off_t to_write;
+		if (offset + reuse_packfile->bitmap_nr < BITS_IN_EWORD)
+			last = offset + reuse_packfile->bitmap_nr;
+		else
+			last = BITS_IN_EWORD;

-		written = (pos * BITS_IN_EWORD);
-		to_write = pack_pos_to_offset(reuse_packfile, written)
-			- sizeof(struct pack_header);
+		for (; offset < last; offset++) {
+			if (word >> offset == 0)
+				return word_pos;
+			if (!bitmap_get(reuse_packfile_bitmap,
+					word_pos * BITS_IN_EWORD + offset))
+				return word_pos;
+		}
+
+		pos += BITS_IN_EWORD - (pos % BITS_IN_EWORD);
+	}
+
+	/*
+	 * Now we're going to copy as many whole eword_t's as possible.
+	 * "end" is the index of the last whole eword_t we copy, but
+	 * there may be additional bits to process. Those are handled
+	 * individually by write_reused_pack().
+	 *
+	 * Begin by advancing to the first word boundary in range of the
+	 * bit positions occupied by objects in "reuse_packfile". Then
+	 * pick the last word boundary in the same range. If we have at
+	 * least one word's worth of bits to process, continue on.
+	 */
+	end = reuse_packfile->bitmap_pos + reuse_packfile->bitmap_nr;
+	if (end % BITS_IN_EWORD)
+		end -= end % BITS_IN_EWORD;
+	if (pos >= end)
+		return reuse_packfile->bitmap_pos / BITS_IN_EWORD;
+
+	while (pos < end &&
+	       reuse_packfile_bitmap->words[pos / BITS_IN_EWORD] == (eword_t)~0)
+		pos += BITS_IN_EWORD;
+
+	if (pos > end)
+		pos = end;
+
+	if (reuse_packfile->bitmap_pos < pos) {
+		off_t pack_start_off = pack_pos_to_offset(reuse_packfile->p, 0);
+		off_t pack_end_off = pack_pos_to_offset(reuse_packfile->p,
+							pos - reuse_packfile->bitmap_pos);
+
+		written += pos - reuse_packfile->bitmap_pos;

 		/* We're recording one chunk, not one object. */
-		record_reused_object(sizeof(struct pack_header), 0);
+		record_reused_object(pack_start_off,
+				     pack_start_off - (hashfile_total(out) - pack_start));
 		hashflush(out);
-		copy_pack_data(out, reuse_packfile, w_curs,
-			sizeof(struct pack_header), to_write);
+		copy_pack_data(out, reuse_packfile->p, w_curs,
+			pack_start_off, pack_end_off - pack_start_off);

 		display_progress(progress_state, written);
 	}
-	return pos;
+	if (pos % BITS_IN_EWORD)
+		BUG("attempted to jump past a word boundary to %"PRIuMAX,
+		    (uintmax_t)pos);
+	return pos / BITS_IN_EWORD;
 }

-static void write_reused_pack(struct hashfile *f)
+static void write_reused_pack(struct bitmapped_pack *reuse_packfile,
+			      struct hashfile *f)
 {
-	size_t i = 0;
+	size_t i = reuse_packfile->bitmap_pos / BITS_IN_EWORD;
 	uint32_t offset;
+	off_t pack_start = hashfile_total(f) - sizeof(struct pack_header);
 	struct pack_window *w_curs = NULL;

 	if (allow_ofs_delta)
-		i = write_reused_pack_verbatim(f, &w_curs);
+		i = write_reused_pack_verbatim(reuse_packfile, f, pack_start,
+					       &w_curs);

 	for (; i < reuse_packfile_bitmap->word_alloc; ++i) {
 		eword_t word = reuse_packfile_bitmap->words[i];
@ -1133,16 +1194,23 @@ static void write_reused_pack(struct hashfile *f)
 				break;

 			offset += ewah_bit_ctz64(word >> offset);
+			if (pos + offset < reuse_packfile->bitmap_pos)
+				continue;
+			if (pos + offset >= reuse_packfile->bitmap_pos + reuse_packfile->bitmap_nr)
+				goto done;
 			/*
 			 * Can use bit positions directly, even for MIDX
 			 * bitmaps. See comment in try_partial_reuse()
 			 * for why.
 			 */
-			write_reused_pack_one(pos + offset, f, &w_curs);
+			write_reused_pack_one(reuse_packfile->p,
+					      pos + offset - reuse_packfile->bitmap_pos,
+					      f, pack_start, &w_curs);
 			display_progress(progress_state, ++written);
 		}
 	}

+done:
 	unuse_pack(&w_curs);
 }

@ -1194,9 +1262,14 @@ static void write_pack_file(void)

 		offset = write_pack_header(f, nr_remaining);

-		if (reuse_packfile) {
+		if (reuse_packfiles_nr) {
 			assert(pack_to_stdout);
-			write_reused_pack(f);
+			for (j = 0; j < reuse_packfiles_nr; j++) {
+				reused_chunks_nr = 0;
+				write_reused_pack(&reuse_packfiles[j], f);
+				if (reused_chunks_nr)
+					reuse_packfiles_used_nr++;
+			}
 			offset = hashfile_total(f);
 		}

@ -3172,7 +3245,19 @@ static int git_pack_config(const char *k, const char *v,
 		return 0;
 	}
 	if (!strcmp(k, "pack.allowpackreuse")) {
-		allow_pack_reuse = git_config_bool(k, v);
+		int res = git_parse_maybe_bool_text(v);
+		if (res < 0) {
+			if (!strcasecmp(v, "single"))
+				allow_pack_reuse = SINGLE_PACK_REUSE;
+			else if (!strcasecmp(v, "multi"))
+				allow_pack_reuse = MULTI_PACK_REUSE;
+			else
+				die(_("invalid pack.allowPackReuse value: '%s'"), v);
+		} else if (res) {
+			allow_pack_reuse = SINGLE_PACK_REUSE;
+		} else {
+			allow_pack_reuse = NO_PACK_REUSE;
+		}
 		return 0;
 	}
 	if (!strcmp(k, "pack.threads")) {
@ -3931,7 +4016,7 @@ static void loosen_unused_packed_objects(void)
 */
 static int pack_options_allow_reuse(void)
 {
-	return allow_pack_reuse &&
+	return allow_pack_reuse != NO_PACK_REUSE &&
 	       pack_to_stdout &&
 	       !ignore_packed_keep_on_disk &&
 	       !ignore_packed_keep_in_core &&
@ -3944,13 +4029,18 @@ static int get_object_list_from_bitmap(struct rev_info *revs)
 	if (!(bitmap_git = prepare_bitmap_walk(revs, 0)))
 		return -1;

-	if (pack_options_allow_reuse() &&
-	    !reuse_partial_packfile_from_bitmap(
-			bitmap_git,
-			&reuse_packfile,
-			&reuse_packfile_objects,
-			&reuse_packfile_bitmap)) {
-		assert(reuse_packfile_objects);
+	if (pack_options_allow_reuse())
+		reuse_partial_packfile_from_bitmap(bitmap_git,
+						   &reuse_packfiles,
+						   &reuse_packfiles_nr,
+						   &reuse_packfile_bitmap,
+						   allow_pack_reuse == MULTI_PACK_REUSE);
+
+	if (reuse_packfiles) {
+		reuse_packfile_objects = bitmap_popcount(reuse_packfile_bitmap);
+		if (!reuse_packfile_objects)
+			BUG("expected non-empty reuse bitmap");
+
 		nr_result += reuse_packfile_objects;
 		nr_seen += reuse_packfile_objects;
 		display_progress(progress_state, nr_seen);
@ -4518,11 +4608,20 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 		fprintf_ln(stderr,
 			   _("Total %"PRIu32" (delta %"PRIu32"),"
 			     " reused %"PRIu32" (delta %"PRIu32"),"
-			     " pack-reused %"PRIu32),
+			     " pack-reused %"PRIu32" (from %"PRIuMAX")"),
 			   written, written_delta, reused, reused_delta,
-			   reuse_packfile_objects);
+			   reuse_packfile_objects,
+			   (uintmax_t)reuse_packfiles_used_nr);
+
+	trace2_data_intmax("pack-objects", the_repository, "written", written);
+	trace2_data_intmax("pack-objects", the_repository, "written/delta", written_delta);
+	trace2_data_intmax("pack-objects", the_repository, "reused", reused);
+	trace2_data_intmax("pack-objects", the_repository, "reused/delta", reused_delta);
+	trace2_data_intmax("pack-objects", the_repository, "pack-reused", reuse_packfile_objects);
+	trace2_data_intmax("pack-objects", the_repository, "packs-reused", reuse_packfiles_used_nr);

 cleanup:
+	clear_packing_data(&to_pack);
 	list_objects_filter_release(&filter_options);
 	strvec_clear(&rp);

--- a/ewah/bitmap.c
+++ b/ewah/bitmap.c
@ -169,6 +169,15 @@ size_t bitmap_popcount(struct bitmap *self)
 	return count;
 }

+int bitmap_is_empty(struct bitmap *self)
+{
+	size_t i;
+	for (i = 0; i < self->word_alloc; i++)
+		if (self->words[i])
+			return 0;
+	return 1;
+}
+
 int bitmap_equals(struct bitmap *self, struct bitmap *other)
 {
 	struct bitmap *big, *small;
--- a/ewah/ewok.h
+++ b/ewah/ewok.h
@ -189,5 +189,6 @@ void bitmap_or_ewah(struct bitmap *self, struct ewah_bitmap *other);
 void bitmap_or(struct bitmap *self, const struct bitmap *other);

 size_t bitmap_popcount(struct bitmap *self);
+int bitmap_is_empty(struct bitmap *self);

 #endif
--- a/git-compat-util.h
+++ b/git-compat-util.h
@ -1015,6 +1015,15 @@ static inline unsigned long cast_size_t_to_ulong(size_t a)
 	return (unsigned long)a;
 }

+static inline uint32_t cast_size_t_to_uint32_t(size_t a)
+{
+	if (a != (uint32_t)a)
+		die("object too large to read on this platform: %"
+		    PRIuMAX" is cut off to %u",
+		    (uintmax_t)a, (uint32_t)a);
+	return (uint32_t)a;
+}
+
 static inline int cast_size_t_to_int(size_t a)
 {
 	if (a > INT_MAX)
--- a/midx.c
+++ b/midx.c
@ -21,6 +21,7 @@
 #include "refs.h"
 #include "revision.h"
 #include "list-objects.h"
+#include "pack-revindex.h"

 #define MIDX_SIGNATURE 0x4d494458 /* "MIDX" */
 #define MIDX_VERSION 1
@ -33,6 +34,7 @@

 #define MIDX_CHUNK_ALIGNMENT 4
 #define MIDX_CHUNKID_PACKNAMES 0x504e414d /* "PNAM" */
+#define MIDX_CHUNKID_BITMAPPEDPACKS 0x42544d50 /* "BTMP" */
 #define MIDX_CHUNKID_OIDFANOUT 0x4f494446 /* "OIDF" */
 #define MIDX_CHUNKID_OIDLOOKUP 0x4f49444c /* "OIDL" */
 #define MIDX_CHUNKID_OBJECTOFFSETS 0x4f4f4646 /* "OOFF" */
@ -41,6 +43,7 @@
 #define MIDX_CHUNK_FANOUT_SIZE (sizeof(uint32_t) * 256)
 #define MIDX_CHUNK_OFFSET_WIDTH (2 * sizeof(uint32_t))
 #define MIDX_CHUNK_LARGE_OFFSET_WIDTH (sizeof(uint64_t))
+#define MIDX_CHUNK_BITMAPPED_PACKS_WIDTH (2 * sizeof(uint32_t))
 #define MIDX_LARGE_OFFSET_NEEDED 0x80000000

 #define PACK_EXPIRED UINT_MAX
@ -175,6 +178,8 @@ struct multi_pack_index *load_multi_pack_index(const char *object_dir, int local

 	m->num_packs = get_be32(m->data + MIDX_BYTE_NUM_PACKS);

+	m->preferred_pack_idx = -1;
+
 	cf = init_chunkfile(NULL);

 	if (read_table_of_contents(cf, m->data, midx_size,
@ -193,6 +198,9 @@ struct multi_pack_index *load_multi_pack_index(const char *object_dir, int local

 	pair_chunk(cf, MIDX_CHUNKID_LARGEOFFSETS, &m->chunk_large_offsets,
 		   &m->chunk_large_offsets_len);
+	pair_chunk(cf, MIDX_CHUNKID_BITMAPPEDPACKS,
+		   (const unsigned char **)&m->chunk_bitmapped_packs,
+		   &m->chunk_bitmapped_packs_len);

 	if (git_env_bool("GIT_TEST_MIDX_READ_RIDX", 1))
 		pair_chunk(cf, MIDX_CHUNKID_REVINDEX, &m->chunk_revindex,
@ -286,6 +294,26 @@ int prepare_midx_pack(struct repository *r, struct multi_pack_index *m, uint32_t
 	return 0;
 }

+int nth_bitmapped_pack(struct repository *r, struct multi_pack_index *m,
+		       struct bitmapped_pack *bp, uint32_t pack_int_id)
+{
+	if (!m->chunk_bitmapped_packs)
+		return error(_("MIDX does not contain the BTMP chunk"));
+
+	if (prepare_midx_pack(r, m, pack_int_id))
+		return error(_("could not load bitmapped pack %"PRIu32), pack_int_id);
+
+	bp->p = m->packs[pack_int_id];
+	bp->bitmap_pos = get_be32((char *)m->chunk_bitmapped_packs +
+				  MIDX_CHUNK_BITMAPPED_PACKS_WIDTH * pack_int_id);
+	bp->bitmap_nr = get_be32((char *)m->chunk_bitmapped_packs +
+				 MIDX_CHUNK_BITMAPPED_PACKS_WIDTH * pack_int_id +
+				 sizeof(uint32_t));
+	bp->pack_int_id = pack_int_id;
+
+	return 0;
+}
+
 int bsearch_midx(const struct object_id *oid, struct multi_pack_index *m, uint32_t *result)
 {
 	return bsearch_hash(oid->hash, m->chunk_oid_fanout, m->chunk_oid_lookup,
@ -403,7 +431,8 @@ static int cmp_idx_or_pack_name(const char *idx_or_pack_name,
 	return strcmp(idx_or_pack_name, idx_name);
 }

-int midx_contains_pack(struct multi_pack_index *m, const char *idx_or_pack_name)
+int midx_locate_pack(struct multi_pack_index *m, const char *idx_or_pack_name,
+		     uint32_t *pos)
 {
 	uint32_t first = 0, last = m->num_packs;

@ -414,8 +443,11 @@ int midx_contains_pack(struct multi_pack_index *m, const char *idx_or_pack_name)

 		current = m->pack_names[mid];
 		cmp = cmp_idx_or_pack_name(idx_or_pack_name, current);
-		if (!cmp)
+		if (!cmp) {
+			if (pos)
+				*pos = mid;
 			return 1;
+		}
 		if (cmp > 0) {
 			first = mid + 1;
 			continue;
@ -426,6 +458,28 @@ int midx_contains_pack(struct multi_pack_index *m, const char *idx_or_pack_name)
 	return 0;
 }

+int midx_contains_pack(struct multi_pack_index *m, const char *idx_or_pack_name)
+{
+	return midx_locate_pack(m, idx_or_pack_name, NULL);
+}
+
+int midx_preferred_pack(struct multi_pack_index *m, uint32_t *pack_int_id)
+{
+	if (m->preferred_pack_idx == -1) {
+		if (load_midx_revindex(m) < 0) {
+			m->preferred_pack_idx = -2;
+			return -1;
+		}
+
+		m->preferred_pack_idx =
+			nth_midxed_pack_int_id(m, pack_pos_to_midx(m, 0));
+	} else if (m->preferred_pack_idx == -2)
+		return -1; /* no revindex */
+
+	*pack_int_id = m->preferred_pack_idx;
+	return 0;
+}
+
 int prepare_multi_pack_index_one(struct repository *r, const char *object_dir, int local)
 {
 	struct multi_pack_index *m;
@ -468,13 +522,31 @@ static size_t write_midx_header(struct hashfile *f,
 	return MIDX_HEADER_SIZE;
 }

+#define BITMAP_POS_UNKNOWN (~((uint32_t)0))
+
 struct pack_info {
 	uint32_t orig_pack_int_id;
 	char *pack_name;
 	struct packed_git *p;
+
+	uint32_t bitmap_pos;
+	uint32_t bitmap_nr;
+
 	unsigned expired : 1;
 };

+static void fill_pack_info(struct pack_info *info,
+			   struct packed_git *p, const char *pack_name,
+			   uint32_t orig_pack_int_id)
+{
+	memset(info, 0, sizeof(struct pack_info));
+
+	info->orig_pack_int_id = orig_pack_int_id;
+	info->pack_name = xstrdup(pack_name);
+	info->p = p;
+	info->bitmap_pos = BITMAP_POS_UNKNOWN;
+}
+
 static int pack_info_compare(const void *_a, const void *_b)
 {
 	struct pack_info *a = (struct pack_info *)_a;
@ -515,6 +587,7 @@ static void add_pack_to_midx(const char *full_path, size_t full_path_len,
 			     const char *file_name, void *data)
 {
 	struct write_midx_context *ctx = data;
+	struct packed_git *p;

 	if (ends_with(file_name, ".idx")) {
 		display_progress(ctx->progress, ++ctx->pack_paths_checked);
@ -541,27 +614,22 @@ static void add_pack_to_midx(const char *full_path, size_t full_path_len,

 		ALLOC_GROW(ctx->info, ctx->nr + 1, ctx->alloc);

-		ctx->info[ctx->nr].p = add_packed_git(full_path,
-						      full_path_len,
-						      0);
-
-		if (!ctx->info[ctx->nr].p) {
+		p = add_packed_git(full_path, full_path_len, 0);
+		if (!p) {
 			warning(_("failed to add packfile '%s'"),
 				full_path);
 			return;
 		}

-		if (open_pack_index(ctx->info[ctx->nr].p)) {
+		if (open_pack_index(p)) {
 			warning(_("failed to open pack-index '%s'"),
 				full_path);
-			close_pack(ctx->info[ctx->nr].p);
-			FREE_AND_NULL(ctx->info[ctx->nr].p);
+			close_pack(p);
+			free(p);
 			return;
 		}

-		ctx->info[ctx->nr].pack_name = xstrdup(file_name);
-		ctx->info[ctx->nr].orig_pack_int_id = ctx->nr;
-		ctx->info[ctx->nr].expired = 0;
+		fill_pack_info(&ctx->info[ctx->nr], p, file_name, ctx->nr);
 		ctx->nr++;
 	}
 }
@ -817,6 +885,26 @@ static int write_midx_pack_names(struct hashfile *f, void *data)
 	return 0;
 }

+static int write_midx_bitmapped_packs(struct hashfile *f, void *data)
+{
+	struct write_midx_context *ctx = data;
+	size_t i;
+
+	for (i = 0; i < ctx->nr; i++) {
+		struct pack_info *pack = &ctx->info[i];
+		if (pack->expired)
+			continue;
+
+		if (pack->bitmap_pos == BITMAP_POS_UNKNOWN && pack->bitmap_nr)
+			BUG("pack '%s' has no bitmap position, but has %d bitmapped object(s)",
+			    pack->pack_name, pack->bitmap_nr);
+
+		hashwrite_be32(f, pack->bitmap_pos);
+		hashwrite_be32(f, pack->bitmap_nr);
+	}
+	return 0;
+}
+
 static int write_midx_oid_fanout(struct hashfile *f,
 				 void *data)
 {
@ -984,8 +1072,19 @@ static uint32_t *midx_pack_order(struct write_midx_context *ctx)
 	QSORT(data, ctx->entries_nr, midx_pack_order_cmp);

 	ALLOC_ARRAY(pack_order, ctx->entries_nr);
-	for (i = 0; i < ctx->entries_nr; i++)
+	for (i = 0; i < ctx->entries_nr; i++) {
+		struct pack_midx_entry *e = &ctx->entries[data[i].nr];
+		struct pack_info *pack = &ctx->info[ctx->pack_perm[e->pack_int_id]];
+		if (pack->bitmap_pos == BITMAP_POS_UNKNOWN)
+			pack->bitmap_pos = i;
+		pack->bitmap_nr++;
 		pack_order[i] = data[i].nr;
+	}
+	for (i = 0; i < ctx->nr; i++) {
+		struct pack_info *pack = &ctx->info[ctx->pack_perm[i]];
+		if (pack->bitmap_pos == BITMAP_POS_UNKNOWN)
+			pack->bitmap_pos = 0;
+	}
 	free(data);

 	trace2_region_leave("midx", "midx_pack_order", the_repository);
@ -1286,6 +1385,7 @@ static int write_midx_internal(const char *object_dir,
 	struct hashfile *f = NULL;
 	struct lock_file lk;
 	struct write_midx_context ctx = { 0 };
+	int bitmapped_packs_concat_len = 0;
 	int pack_name_concat_len = 0;
 	int dropped_packs = 0;
 	int result = 0;
@ -1321,11 +1421,6 @@ static int write_midx_internal(const char *object_dir,
 		for (i = 0; i < ctx.m->num_packs; i++) {
 			ALLOC_GROW(ctx.info, ctx.nr + 1, ctx.alloc);

-			ctx.info[ctx.nr].orig_pack_int_id = i;
-			ctx.info[ctx.nr].pack_name = xstrdup(ctx.m->pack_names[i]);
-			ctx.info[ctx.nr].p = ctx.m->packs[i];
-			ctx.info[ctx.nr].expired = 0;
-
 			if (flags & MIDX_WRITE_REV_INDEX) {
 				/*
 				 * If generating a reverse index, need to have
@ -1341,10 +1436,10 @@ static int write_midx_internal(const char *object_dir,
 				if (open_pack_index(ctx.m->packs[i]))
 					die(_("could not open index for %s"),
 					    ctx.m->packs[i]->pack_name);
-				ctx.info[ctx.nr].p = ctx.m->packs[i];
 			}

-			ctx.nr++;
+			fill_pack_info(&ctx.info[ctx.nr++], ctx.m->packs[i],
+				       ctx.m->pack_names[i], i);
 		}
 	}

@ -1503,8 +1598,10 @@ static int write_midx_internal(const char *object_dir,
 	}

 	for (i = 0; i < ctx.nr; i++) {
-		if (!ctx.info[i].expired)
-			pack_name_concat_len += strlen(ctx.info[i].pack_name) + 1;
+		if (ctx.info[i].expired)
+			continue;
+		pack_name_concat_len += strlen(ctx.info[i].pack_name) + 1;
+		bitmapped_packs_concat_len += 2 * sizeof(uint32_t);
 	}

 	/* Check that the preferred pack wasn't expired (if given). */
@ -1564,6 +1661,9 @@ static int write_midx_internal(const char *object_dir,
 		add_chunk(cf, MIDX_CHUNKID_REVINDEX,
 			  st_mult(ctx.entries_nr, sizeof(uint32_t)),
 			  write_midx_revindex);
+		add_chunk(cf, MIDX_CHUNKID_BITMAPPEDPACKS,
+			  bitmapped_packs_concat_len,
+			  write_midx_bitmapped_packs);
 	}

 	write_midx_header(f, get_num_chunks(cf), ctx.nr - dropped_packs);
@ -1603,8 +1703,13 @@ static int write_midx_internal(const char *object_dir,
 				      flags) < 0) {
 			error(_("could not write multi-pack bitmap"));
 			result = 1;
+			clear_packing_data(&pdata);
+			free(commits);
 			goto cleanup;
 		}
+
+		clear_packing_data(&pdata);
+		free(commits);
 	}
 	/*
 	 * NOTE: Do not use ctx.entries beyond this point, since it might
--- a/midx.h
+++ b/midx.h
@ -6,6 +6,7 @@
 struct object_id;
 struct pack_entry;
 struct repository;
+struct bitmapped_pack;

 #define GIT_TEST_MULTI_PACK_INDEX "GIT_TEST_MULTI_PACK_INDEX"
 #define GIT_TEST_MULTI_PACK_INDEX_WRITE_BITMAP \
@ -27,11 +28,14 @@ struct multi_pack_index {
 	unsigned char num_chunks;
 	uint32_t num_packs;
 	uint32_t num_objects;
+	int preferred_pack_idx;

 	int local;

 	const unsigned char *chunk_pack_names;
 	size_t chunk_pack_names_len;
+	const uint32_t *chunk_bitmapped_packs;
+	size_t chunk_bitmapped_packs_len;
 	const uint32_t *chunk_oid_fanout;
 	const unsigned char *chunk_oid_lookup;
 	const unsigned char *chunk_object_offsets;
@ -57,6 +61,8 @@ void get_midx_rev_filename(struct strbuf *out, struct multi_pack_index *m);

 struct multi_pack_index *load_multi_pack_index(const char *object_dir, int local);
 int prepare_midx_pack(struct repository *r, struct multi_pack_index *m, uint32_t pack_int_id);
+int nth_bitmapped_pack(struct repository *r, struct multi_pack_index *m,
+		       struct bitmapped_pack *bp, uint32_t pack_int_id);
 int bsearch_midx(const struct object_id *oid, struct multi_pack_index *m, uint32_t *result);
 off_t nth_midxed_offset(struct multi_pack_index *m, uint32_t pos);
 uint32_t nth_midxed_pack_int_id(struct multi_pack_index *m, uint32_t pos);
@ -64,7 +70,11 @@ struct object_id *nth_midxed_object_oid(struct object_id *oid,
 					struct multi_pack_index *m,
 					uint32_t n);
 int fill_midx_entry(struct repository *r, const struct object_id *oid, struct pack_entry *e, struct multi_pack_index *m);
-int midx_contains_pack(struct multi_pack_index *m, const char *idx_or_pack_name);
+int midx_contains_pack(struct multi_pack_index *m,
+		       const char *idx_or_pack_name);
+int midx_locate_pack(struct multi_pack_index *m, const char *idx_or_pack_name,
+		     uint32_t *pos);
+int midx_preferred_pack(struct multi_pack_index *m, uint32_t *pack_int_id);
 int prepare_multi_pack_index_one(struct repository *r, const char *object_dir, int local);

 /*
--- a/pack-bitmap-write.c
+++ b/pack-bitmap-write.c
@ -195,6 +195,13 @@ struct bb_commit {
 	unsigned idx; /* within selected array */
 };

+static void clear_bb_commit(struct bb_commit *commit)
+{
+	free_commit_list(commit->reverse_edges);
+	bitmap_free(commit->commit_mask);
+	bitmap_free(commit->bitmap);
+}
+
 define_commit_slab(bb_data, struct bb_commit);

 struct bitmap_builder {
@ -336,7 +343,7 @@ next:

 static void bitmap_builder_clear(struct bitmap_builder *bb)
 {
-	clear_bb_data(&bb->data);
+	deep_clear_bb_data(&bb->data, clear_bb_commit);
 	free(bb->commits);
 	bb->commits_nr = bb->commits_alloc = 0;
 }
--- a/pack-bitmap.c
+++ b/pack-bitmap.c
@ -338,7 +338,7 @@ static int open_midx_bitmap_1(struct bitmap_index *bitmap_git,
 	struct stat st;
 	char *bitmap_name = midx_bitmap_filename(midx);
 	int fd = git_open(bitmap_name);
-	uint32_t i;
+	uint32_t i, preferred_pack;
 	struct packed_git *preferred;

 	if (fd < 0) {
@ -393,7 +393,12 @@ static int open_midx_bitmap_1(struct bitmap_index *bitmap_git,
 		}
 	}

-	preferred = bitmap_git->midx->packs[midx_preferred_pack(bitmap_git)];
+	if (midx_preferred_pack(bitmap_git->midx, &preferred_pack) < 0) {
+		warning(_("could not determine MIDX preferred pack"));
+		goto cleanup;
+	}
+
+	preferred = bitmap_git->midx->packs[preferred_pack];
 	if (!is_pack_valid(preferred)) {
 		warning(_("preferred pack (%s) is invalid"),
 			preferred->pack_name);
@ -1280,6 +1285,8 @@ static struct bitmap *find_objects(struct bitmap_index *bitmap_git,
 		base = fill_in_bitmap(bitmap_git, revs, base, seen);
 	}

+	object_list_free(&not_mapped);
+
 	return base;
 }

@ -1834,8 +1841,10 @@ cleanup:
 * -1 means "stop trying further objects"; 0 means we may or may not have
 * reused, but you can keep feeding bits.
 */
-static int try_partial_reuse(struct packed_git *pack,
-			     size_t pos,
+static int try_partial_reuse(struct bitmap_index *bitmap_git,
+			     struct bitmapped_pack *pack,
+			     size_t bitmap_pos,
+			     uint32_t pack_pos,
 			     struct bitmap *reuse,
 			     struct pack_window **w_curs)
 {
@ -1843,40 +1852,18 @@ static int try_partial_reuse(struct packed_git *pack,
 	enum object_type type;
 	unsigned long size;

-	/*
-	 * try_partial_reuse() is called either on (a) objects in the
-	 * bitmapped pack (in the case of a single-pack bitmap) or (b)
-	 * objects in the preferred pack of a multi-pack bitmap.
-	 * Importantly, the latter can pretend as if only a single pack
-	 * exists because:
-	 *
-	 *   - The first pack->num_objects bits of a MIDX bitmap are
-	 *     reserved for the preferred pack, and
-	 *
-	 *   - Ties due to duplicate objects are always resolved in
-	 *     favor of the preferred pack.
-	 *
-	 * Therefore we do not need to ever ask the MIDX for its copy of
-	 * an object by OID, since it will always select it from the
-	 * preferred pack. Likewise, the selected copy of the base
-	 * object for any deltas will reside in the same pack.
-	 *
-	 * This means that we can reuse pos when looking up the bit in
-	 * the reuse bitmap, too, since bits corresponding to the
-	 * preferred pack precede all bits from other packs.
-	 */
+	if (pack_pos >= pack->p->num_objects)
+		return -1; /* not actually in the pack */

-	if (pos >= pack->num_objects)
-		return -1; /* not actually in the pack or MIDX preferred pack */
-
-	offset = delta_obj_offset = pack_pos_to_offset(pack, pos);
-	type = unpack_object_header(pack, w_curs, &offset, &size);
+	offset = delta_obj_offset = pack_pos_to_offset(pack->p, pack_pos);
+	type = unpack_object_header(pack->p, w_curs, &offset, &size);
 	if (type < 0)
 		return -1; /* broken packfile, punt */

 	if (type == OBJ_REF_DELTA || type == OBJ_OFS_DELTA) {
 		off_t base_offset;
 		uint32_t base_pos;
+		uint32_t base_bitmap_pos;

 		/*
 		 * Find the position of the base object so we can look it up
@ -1886,24 +1873,48 @@ static int try_partial_reuse(struct packed_git *pack,
 		 * and the normal slow path will complain about it in
 		 * more detail.
 		 */
-		base_offset = get_delta_base(pack, w_curs, &offset, type,
+		base_offset = get_delta_base(pack->p, w_curs, &offset, type,
 					     delta_obj_offset);
 		if (!base_offset)
 			return 0;
-		if (offset_to_pack_pos(pack, base_offset, &base_pos) < 0)
-			return 0;

-		/*
-		 * We assume delta dependencies always point backwards. This
-		 * lets us do a single pass, and is basically always true
-		 * due to the way OFS_DELTAs work. You would not typically
-		 * find REF_DELTA in a bitmapped pack, since we only bitmap
-		 * packs we write fresh, and OFS_DELTA is the default). But
-		 * let's double check to make sure the pack wasn't written with
-		 * odd parameters.
-		 */
-		if (base_pos >= pos)
-			return 0;
+		offset_to_pack_pos(pack->p, base_offset, &base_pos);
+
+		if (bitmap_is_midx(bitmap_git)) {
+			/*
+			 * Cross-pack deltas are rejected for now, but could
+			 * theoretically be supported in the future.
+			 *
+			 * We would need to ensure that we're sending both
+			 * halves of the delta/base pair, regardless of whether
+			 * or not the two cross a pack boundary. If they do,
+			 * then we must convert the delta to an REF_DELTA to
+			 * refer back to the base in the other pack.
+			 * */
+			if (midx_pair_to_pack_pos(bitmap_git->midx,
+						  pack->pack_int_id,
+						  base_offset,
+						  &base_bitmap_pos) < 0) {
+				return 0;
+			}
+		} else {
+			if (offset_to_pack_pos(pack->p, base_offset,
+					       &base_pos) < 0)
+				return 0;
+			/*
+			 * We assume delta dependencies always point backwards.
+			 * This lets us do a single pass, and is basically
+			 * always true due to the way OFS_DELTAs work. You would
+			 * not typically find REF_DELTA in a bitmapped pack,
+			 * since we only bitmap packs we write fresh, and
+			 * OFS_DELTA is the default). But let's double check to
+			 * make sure the pack wasn't written with odd
+			 * parameters.
+			 */
+			if (base_pos >= pack_pos)
+				return 0;
+			base_bitmap_pos = pack->bitmap_pos + base_pos;
+		}

 		/*
 		 * And finally, if we're not sending the base as part of our
@ -1913,77 +1924,89 @@ static int try_partial_reuse(struct packed_git *pack,
 		 * to REF_DELTA on the fly. Better to just let the normal
 		 * object_entry code path handle it.
 		 */
-		if (!bitmap_get(reuse, base_pos))
+		if (!bitmap_get(reuse, base_bitmap_pos))
 			return 0;
 	}

 	/*
 	 * If we got here, then the object is OK to reuse. Mark it.
 	 */
-	bitmap_set(reuse, pos);
+	bitmap_set(reuse, bitmap_pos);
 	return 0;
 }

-uint32_t midx_preferred_pack(struct bitmap_index *bitmap_git)
+static void reuse_partial_packfile_from_bitmap_1(struct bitmap_index *bitmap_git,
+						 struct bitmapped_pack *pack,
+						 struct bitmap *reuse)
 {
-	struct multi_pack_index *m = bitmap_git->midx;
-	if (!m)
-		BUG("midx_preferred_pack: requires non-empty MIDX");
-	return nth_midxed_pack_int_id(m, pack_pos_to_midx(bitmap_git->midx, 0));
-}
-
-int reuse_partial_packfile_from_bitmap(struct bitmap_index *bitmap_git,
-				       struct packed_git **packfile_out,
-				       uint32_t *entries,
-				       struct bitmap **reuse_out)
-{
-	struct repository *r = the_repository;
-	struct packed_git *pack;
 	struct bitmap *result = bitmap_git->result;
-	struct bitmap *reuse;
 	struct pack_window *w_curs = NULL;
-	size_t i = 0;
-	uint32_t offset;
-	uint32_t objects_nr;
+	size_t pos = pack->bitmap_pos / BITS_IN_EWORD;

-	assert(result);
+	if (!pack->bitmap_pos) {
+		/*
+		 * If we're processing the first (in the case of a MIDX, the
+		 * preferred pack) or the only (in the case of single-pack
+		 * bitmaps) pack, then we can reuse whole words at a time.
+		 *
+		 * This is because we know that any deltas in this range *must*
+		 * have their bases chosen from the same pack, since:
+		 *
+		 * - In the single pack case, there is no other pack to choose
+		 *   them from.
+		 *
+		 * - In the MIDX case, the first pack is the preferred pack, so
+		 *   all ties are broken in favor of that pack (i.e. the one
+		 *   we're currently processing). So any duplicate bases will be
+		 *   resolved in favor of the pack we're processing.
+		 */
+		while (pos < result->word_alloc &&
+		       pos < pack->bitmap_nr / BITS_IN_EWORD &&
+		       result->words[pos] == (eword_t)~0)
+			pos++;
+		memset(reuse->words, 0xFF, pos * sizeof(eword_t));
+	}

-	load_reverse_index(r, bitmap_git);
+	for (; pos < result->word_alloc; pos++) {
+		eword_t word = result->words[pos];
+		size_t offset;

-	if (bitmap_is_midx(bitmap_git))
-		pack = bitmap_git->midx->packs[midx_preferred_pack(bitmap_git)];
-	else
-		pack = bitmap_git->pack;
-	objects_nr = pack->num_objects;
+		for (offset = 0; offset < BITS_IN_EWORD; offset++) {
+			size_t bit_pos;
+			uint32_t pack_pos;

-	while (i < result->word_alloc && result->words[i] == (eword_t)~0)
-		i++;
-
-	/*
-	 * Don't mark objects not in the packfile or preferred pack. This bitmap
-	 * marks objects eligible for reuse, but the pack-reuse code only
-	 * understands how to reuse a single pack. Since the preferred pack is
-	 * guaranteed to have all bases for its deltas (in a multi-pack bitmap),
-	 * we use it instead of another pack. In single-pack bitmaps, the choice
-	 * is made for us.
-	 */
-	if (i > objects_nr / BITS_IN_EWORD)
-		i = objects_nr / BITS_IN_EWORD;
-
-	reuse = bitmap_word_alloc(i);
-	memset(reuse->words, 0xFF, i * sizeof(eword_t));
-
-	for (; i < result->word_alloc; ++i) {
-		eword_t word = result->words[i];
-		size_t pos = (i * BITS_IN_EWORD);
-
-		for (offset = 0; offset < BITS_IN_EWORD; ++offset) {
-			if ((word >> offset) == 0)
+			if (word >> offset == 0)
 				break;

 			offset += ewah_bit_ctz64(word >> offset);
-			if (try_partial_reuse(pack, pos + offset,
-					      reuse, &w_curs) < 0) {
+
+			bit_pos = pos * BITS_IN_EWORD + offset;
+			if (bit_pos < pack->bitmap_pos)
+				continue;
+			if (bit_pos >= pack->bitmap_pos + pack->bitmap_nr)
+				goto done;
+
+			if (bitmap_is_midx(bitmap_git)) {
+				uint32_t midx_pos;
+				off_t ofs;
+
+				midx_pos = pack_pos_to_midx(bitmap_git->midx, bit_pos);
+				ofs = nth_midxed_offset(bitmap_git->midx, midx_pos);
+
+				if (offset_to_pack_pos(pack->p, ofs, &pack_pos) < 0)
+					BUG("could not find object in pack %s "
+					    "at offset %"PRIuMAX" in MIDX",
+					    pack_basename(pack->p), (uintmax_t)ofs);
+			} else {
+				pack_pos = cast_size_t_to_uint32_t(st_sub(bit_pos, pack->bitmap_pos));
+				if (pack_pos >= pack->p->num_objects)
+					BUG("advanced beyond the end of pack %s (%"PRIuMAX" > %"PRIu32")",
+					    pack_basename(pack->p), (uintmax_t)pack_pos,
+					    pack->p->num_objects);
+			}
+
+			if (try_partial_reuse(bitmap_git, pack, bit_pos,
+					      pack_pos, reuse, &w_curs) < 0) {
 				/*
 				 * try_partial_reuse indicated we couldn't reuse
 				 * any bits, so there is no point in trying more
@ -2000,11 +2023,97 @@ int reuse_partial_packfile_from_bitmap(struct bitmap_index *bitmap_git,

 done:
 	unuse_pack(&w_curs);
+}

-	*entries = bitmap_popcount(reuse);
-	if (!*entries) {
-		bitmap_free(reuse);
+static int bitmapped_pack_cmp(const void *va, const void *vb)
+{
+	const struct bitmapped_pack *a = va;
+	const struct bitmapped_pack *b = vb;
+
+	if (a->bitmap_pos < b->bitmap_pos)
 		return -1;
+	if (a->bitmap_pos > b->bitmap_pos)
+		return 1;
+	return 0;
+}
+
+void reuse_partial_packfile_from_bitmap(struct bitmap_index *bitmap_git,
+					struct bitmapped_pack **packs_out,
+					size_t *packs_nr_out,
+					struct bitmap **reuse_out,
+					int multi_pack_reuse)
+{
+	struct repository *r = the_repository;
+	struct bitmapped_pack *packs = NULL;
+	struct bitmap *result = bitmap_git->result;
+	struct bitmap *reuse;
+	size_t i;
+	size_t packs_nr = 0, packs_alloc = 0;
+	size_t word_alloc;
+	uint32_t objects_nr = 0;
+
+	assert(result);
+
+	load_reverse_index(r, bitmap_git);
+
+	if (bitmap_is_midx(bitmap_git)) {
+		for (i = 0; i < bitmap_git->midx->num_packs; i++) {
+			struct bitmapped_pack pack;
+			if (nth_bitmapped_pack(r, bitmap_git->midx, &pack, i) < 0) {
+				warning(_("unable to load pack: '%s', disabling pack-reuse"),
+					bitmap_git->midx->pack_names[i]);
+				free(packs);
+				return;
+			}
+
+			if (!pack.bitmap_nr)
+				continue;
+
+			if (!multi_pack_reuse && pack.bitmap_pos) {
+				/*
+				 * If we're only reusing a single pack, skip
+				 * over any packs which are not positioned at
+				 * the beginning of the MIDX bitmap.
+				 *
+				 * This is consistent with the existing
+				 * single-pack reuse behavior, which only reuses
+				 * parts of the MIDX's preferred pack.
+				 */
+				continue;
+			}
+
+			ALLOC_GROW(packs, packs_nr + 1, packs_alloc);
+			memcpy(&packs[packs_nr++], &pack, sizeof(pack));
+
+			objects_nr += pack.p->num_objects;
+
+			if (!multi_pack_reuse)
+				break;
+		}
+
+		QSORT(packs, packs_nr, bitmapped_pack_cmp);
+	} else {
+		ALLOC_GROW(packs, packs_nr + 1, packs_alloc);
+
+		packs[packs_nr].p = bitmap_git->pack;
+		packs[packs_nr].bitmap_nr = bitmap_git->pack->num_objects;
+		packs[packs_nr].bitmap_pos = 0;
+
+		objects_nr = packs[packs_nr++].bitmap_nr;
+	}
+
+	word_alloc = objects_nr / BITS_IN_EWORD;
+	if (objects_nr % BITS_IN_EWORD)
+		word_alloc++;
+	reuse = bitmap_word_alloc(word_alloc);
+
+	for (i = 0; i < packs_nr; i++)
+		reuse_partial_packfile_from_bitmap_1(bitmap_git, &packs[i], reuse);
+
+	if (bitmap_is_empty(reuse)) {
+		free(packs);
+		bitmap_free(reuse);
+		return;
 	}

 	/*
@ -2012,9 +2121,9 @@ done:
 	 * need to be handled separately.
 	 */
 	bitmap_and_not(result, reuse);
-	*packfile_out = pack;
+	*packs_out = packs;
+	*packs_nr_out = packs_nr;
 	*reuse_out = reuse;
-	return 0;
 }

 int bitmap_walk_contains(struct bitmap_index *bitmap_git,
--- a/pack-bitmap.h
+++ b/pack-bitmap.h
@ -52,6 +52,15 @@ typedef int (*show_reachable_fn)(

 struct bitmap_index;

+struct bitmapped_pack {
+	struct packed_git *p;
+
+	uint32_t bitmap_pos;
+	uint32_t bitmap_nr;
+
+	uint32_t pack_int_id; /* MIDX only */
+};
+
 struct bitmap_index *prepare_bitmap_git(struct repository *r);
 struct bitmap_index *prepare_midx_bitmap_git(struct multi_pack_index *midx);
 void count_bitmap_commit_list(struct bitmap_index *, uint32_t *commits,
@ -68,11 +77,11 @@ int test_bitmap_hashes(struct repository *r);

 struct bitmap_index *prepare_bitmap_walk(struct rev_info *revs,
 					 int filter_provided_objects);
-uint32_t midx_preferred_pack(struct bitmap_index *bitmap_git);
-int reuse_partial_packfile_from_bitmap(struct bitmap_index *,
-				       struct packed_git **packfile,
-				       uint32_t *entries,
-				       struct bitmap **reuse_out);
+void reuse_partial_packfile_from_bitmap(struct bitmap_index *bitmap_git,
+					struct bitmapped_pack **packs_out,
+					size_t *packs_nr_out,
+					struct bitmap **reuse_out,
+					int multi_pack_reuse);
 int rebuild_existing_bitmaps(struct bitmap_index *, struct packing_data *mapping,
 			     kh_oid_map_t *reused_bitmaps, int show_progress);
 void free_bitmap_index(struct bitmap_index *);
--- a/pack-objects.c
+++ b/pack-objects.c
@ -151,6 +151,21 @@ void prepare_packing_data(struct repository *r, struct packing_data *pdata)
 	init_recursive_mutex(&pdata->odb_lock);
 }

+void clear_packing_data(struct packing_data *pdata)
+{
+	if (!pdata)
+		return;
+
+	free(pdata->cruft_mtime);
+	free(pdata->in_pack);
+	free(pdata->in_pack_by_idx);
+	free(pdata->in_pack_pos);
+	free(pdata->index);
+	free(pdata->layer);
+	free(pdata->objects);
+	free(pdata->tree_depth);
+}
+
 struct object_entry *packlist_alloc(struct packing_data *pdata,
 				    const struct object_id *oid)
 {
--- a/pack-objects.h
+++ b/pack-objects.h
@ -169,6 +169,7 @@ struct packing_data {
 };

 void prepare_packing_data(struct repository *r, struct packing_data *pdata);
+void clear_packing_data(struct packing_data *pdata);

 /* Protect access to object database */
 static inline void packing_data_lock(struct packing_data *pdata)
--- a/pack-revindex.c
+++ b/pack-revindex.c
@ -520,19 +520,12 @@ static int midx_pack_order_cmp(const void *va, const void *vb)
 	return 0;
 }

-int midx_to_pack_pos(struct multi_pack_index *m, uint32_t at, uint32_t *pos)
+static int midx_key_to_pack_pos(struct multi_pack_index *m,
+				struct midx_pack_key *key,
+				uint32_t *pos)
 {
-	struct midx_pack_key key;
 	uint32_t *found;

-	if (!m->revindex_data)
-		BUG("midx_to_pack_pos: reverse index not yet loaded");
-	if (m->num_objects <= at)
-		BUG("midx_to_pack_pos: out-of-bounds object at %"PRIu32, at);
-
-	key.pack = nth_midxed_pack_int_id(m, at);
-	key.offset = nth_midxed_offset(m, at);
-	key.midx = m;
 	/*
 	 * The preferred pack sorts first, so determine its identifier by
 	 * looking at the first object in pseudo-pack order.
@ -542,14 +535,43 @@ int midx_to_pack_pos(struct multi_pack_index *m, uint32_t at, uint32_t *pos)
 	 * implicitly is preferred (and includes all its objects, since ties are
 	 * broken first by pack identifier).
 	 */
-	key.preferred_pack = nth_midxed_pack_int_id(m, pack_pos_to_midx(m, 0));
+	if (midx_preferred_pack(key->midx, &key->preferred_pack) < 0)
+		return error(_("could not determine preferred pack"));

-	found = bsearch(&key, m->revindex_data, m->num_objects,
-			sizeof(*m->revindex_data), midx_pack_order_cmp);
+	found = bsearch(key, m->revindex_data, m->num_objects,
+			sizeof(*m->revindex_data),
+			midx_pack_order_cmp);

 	if (!found)
-		return error("bad offset for revindex");
+		return -1;

 	*pos = found - m->revindex_data;
 	return 0;
 }
+
+int midx_to_pack_pos(struct multi_pack_index *m, uint32_t at, uint32_t *pos)
+{
+	struct midx_pack_key key;
+
+	if (!m->revindex_data)
+		BUG("midx_to_pack_pos: reverse index not yet loaded");
+	if (m->num_objects <= at)
+		BUG("midx_to_pack_pos: out-of-bounds object at %"PRIu32, at);
+
+	key.pack = nth_midxed_pack_int_id(m, at);
+	key.offset = nth_midxed_offset(m, at);
+	key.midx = m;
+
+	return midx_key_to_pack_pos(m, &key, pos);
+}
+
+int midx_pair_to_pack_pos(struct multi_pack_index *m, uint32_t pack_int_id,
+			  off_t ofs, uint32_t *pos)
+{
+	struct midx_pack_key key = {
+		.pack = pack_int_id,
+		.offset = ofs,
+		.midx = m,
+	};
+	return midx_key_to_pack_pos(m, &key, pos);
+}
--- a/pack-revindex.h
+++ b/pack-revindex.h
@ -142,4 +142,7 @@ uint32_t pack_pos_to_midx(struct multi_pack_index *m, uint32_t pos);
 */
 int midx_to_pack_pos(struct multi_pack_index *midx, uint32_t at, uint32_t *pos);

+int midx_pair_to_pack_pos(struct multi_pack_index *midx, uint32_t pack_id,
+			  off_t ofs, uint32_t *pos);
+
 #endif
--- a/t/helper/test-read-midx.c
+++ b/t/helper/test-read-midx.c
@ -6,6 +6,7 @@
 #include "pack-bitmap.h"
 #include "packfile.h"
 #include "setup.h"
+#include "gettext.h"

 static int read_midx_file(const char *object_dir, int show_objects)
 {
@ -79,7 +80,7 @@ static int read_midx_checksum(const char *object_dir)
 static int read_midx_preferred_pack(const char *object_dir)
 {
 	struct multi_pack_index *midx = NULL;
-	struct bitmap_index *bitmap = NULL;
+	uint32_t preferred_pack;

 	setup_git_directory();

@ -87,23 +88,45 @@ static int read_midx_preferred_pack(const char *object_dir)
 	if (!midx)
 		return 1;

-	bitmap = prepare_bitmap_git(the_repository);
-	if (!bitmap)
-		return 1;
-	if (!bitmap_is_midx(bitmap)) {
-		free_bitmap_index(bitmap);
+	if (midx_preferred_pack(midx, &preferred_pack) < 0) {
+		warning(_("could not determine MIDX preferred pack"));
 		return 1;
 	}

-	printf("%s\n", midx->pack_names[midx_preferred_pack(bitmap)]);
-	free_bitmap_index(bitmap);
+	printf("%s\n", midx->pack_names[preferred_pack]);
+	return 0;
+}
+
+static int read_midx_bitmapped_packs(const char *object_dir)
+{
+	struct multi_pack_index *midx = NULL;
+	struct bitmapped_pack pack;
+	uint32_t i;
+
+	setup_git_directory();
+
+	midx = load_multi_pack_index(object_dir, 1);
+	if (!midx)
+		return 1;
+
+	for (i = 0; i < midx->num_packs; i++) {
+		if (nth_bitmapped_pack(the_repository, midx, &pack, i) < 0)
+			return 1;
+
+		printf("%s\n", pack_basename(pack.p));
+		printf("  bitmap_pos: %"PRIuMAX"\n", (uintmax_t)pack.bitmap_pos);
+		printf("  bitmap_nr: %"PRIuMAX"\n", (uintmax_t)pack.bitmap_nr);
+	}
+
+	close_midx(midx);
+
 	return 0;
 }

 int cmd__read_midx(int argc, const char **argv)
 {
 	if (!(argc == 2 || argc == 3))
-		usage("read-midx [--show-objects|--checksum|--preferred-pack] <object-dir>");
+		usage("read-midx [--show-objects|--checksum|--preferred-pack|--bitmap] <object-dir>");

 	if (!strcmp(argv[1], "--show-objects"))
 		return read_midx_file(argv[2], 1);
@ -111,5 +134,7 @@ int cmd__read_midx(int argc, const char **argv)
 		return read_midx_checksum(argv[2]);
 	else if (!strcmp(argv[1], "--preferred-pack"))
 		return read_midx_preferred_pack(argv[2]);
+	else if (!strcmp(argv[1], "--bitmap"))
+		return read_midx_bitmapped_packs(argv[2]);
 	return read_midx_file(argv[1], 0);
 }
--- a/t/perf/p5332-multi-pack-reuse.sh
+++ b/t/perf/p5332-multi-pack-reuse.sh
@ -0,0 +1,81 @@
+#!/bin/sh
+
+test_description='tests pack performance with multi-pack reuse'
+
+. ./perf-lib.sh
+. "${TEST_DIRECTORY}/perf/lib-pack.sh"
+
+packdir=.git/objects/pack
+
+test_perf_large_repo
+
+find_pack () {
+	for idx in $packdir/pack-*.idx
+	do
+		if git show-index <$idx | grep -q "$1"
+		then
+			basename $idx
+		fi || return 1
+	done
+}
+
+repack_into_n_chunks () {
+	git repack -adk &&
+
+	test "$1" -eq 1 && return ||
+
+	find $packdir -type f | sort >packs.before &&
+
+	# partition the repository into $1 chunks of consecutive commits, and
+	# then create $1 packs with the objects reachable from each chunk
+	# (excluding any objects reachable from the previous chunks)
+	sz="$(($(git rev-list --count --all) / $1))"
+	for rev in $(git rev-list --all | awk "NR % $sz == 0" | tac)
+	do
+		pack="$(echo "$rev" | git pack-objects --revs \
+			--honor-pack-keep --delta-base-offset $packdir/pack)" &&
+		touch $packdir/pack-$pack.keep || return 1
+	done
+
+	# grab any remaining objects not packed by the previous step(s)
+	git pack-objects --revs --all --honor-pack-keep --delta-base-offset \
+		$packdir/pack &&
+
+	find $packdir -type f | sort >packs.after &&
+
+	# and install the whole thing
+	for f in $(comm -12 packs.before packs.after)
+	do
+		rm -f "$f" || return 1
+	done
+	rm -fr $packdir/*.keep
+}
+
+for nr_packs in 1 10 100
+do
+	test_expect_success "create $nr_packs-pack scenario" '
+		repack_into_n_chunks $nr_packs
+	'
+
+	test_expect_success "setup bitmaps for $nr_packs-pack scenario" '
+		find $packdir -type f -name "*.idx" | sed -e "s/.*\/\(.*\)$/+\1/g" |
+		git multi-pack-index write --stdin-packs --bitmap \
+			--preferred-pack="$(find_pack $(git rev-parse HEAD))"
+	'
+
+	for reuse in single multi
+	do
+		test_perf "clone for $nr_packs-pack scenario ($reuse-pack reuse)" "
+			git for-each-ref --format='%(objectname)' refs/heads refs/tags >in &&
+			git -c pack.allowPackReuse=$reuse pack-objects \
+				--revs --delta-base-offset --use-bitmap-index \
+				--stdout <in >result
+		"
+
+		test_size "clone size for $nr_packs-pack scenario ($reuse-pack reuse)" '
+			wc -c <result
+		'
+	done
+done
+
+test_done
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@ -1171,4 +1171,39 @@ test_expect_success 'reader notices out-of-bounds fanout' '
 	test_cmp expect err
 '

+test_expect_success 'bitmapped packs are stored via the BTMP chunk' '
+	test_when_finished "rm -fr repo" &&
+	git init repo &&
+	(
+		cd repo &&
+
+		for i in 1 2 3 4 5
+		do
+			test_commit "$i" &&
+			git repack -d || return 1
+		done &&
+
+		find $objdir/pack -type f -name "*.idx" | xargs -n 1 basename |
+		sort >packs &&
+
+		git multi-pack-index write --stdin-packs <packs &&
+		test_must_fail test-tool read-midx --bitmap $objdir 2>err &&
+		cat >expect <<-\EOF &&
+		error: MIDX does not contain the BTMP chunk
+		EOF
+		test_cmp expect err &&
+
+		git multi-pack-index write --stdin-packs --bitmap \
+			--preferred-pack="$(head -n1 <packs)" <packs  &&
+		test-tool read-midx --bitmap $objdir >actual &&
+		for i in $(test_seq $(wc -l <packs))
+		do
+			sed -ne "${i}s/\.idx$/\.pack/p" packs &&
+			echo "  bitmap_pos: $((($i - 1) * 3))" &&
+			echo "  bitmap_nr: 3" || return 1
+		done >expect &&
+		test_cmp expect actual
+	)
+'
+
 test_done
--- a/t/t5332-multi-pack-reuse.sh
+++ b/t/t5332-multi-pack-reuse.sh
@ -0,0 +1,203 @@
+#!/bin/sh
+
+test_description='pack-objects multi-pack reuse'
+
+. ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-bitmap.sh
+
+objdir=.git/objects
+packdir=$objdir/pack
+
+test_pack_reused () {
+	test_trace2_data pack-objects pack-reused "$1"
+}
+
+test_packs_reused () {
+	test_trace2_data pack-objects packs-reused "$1"
+}
+
+
+# pack_position <object> </path/to/pack.idx
+pack_position () {
+	git show-index >objects &&
+	grep "$1" objects | cut -d" " -f1
+}
+
+test_expect_success 'preferred pack is reused for single-pack reuse' '
+	test_config pack.allowPackReuse single &&
+
+	for i in A B
+	do
+		test_commit "$i" &&
+		git repack -d || return 1
+	done &&
+
+	git multi-pack-index write --bitmap &&
+
+	: >trace2.txt &&
+	GIT_TRACE2_EVENT="$PWD/trace2.txt" \
+		git pack-objects --stdout --revs --all >/dev/null &&
+
+	test_pack_reused 3 <trace2.txt &&
+	test_packs_reused 1 <trace2.txt
+'
+
+test_expect_success 'enable multi-pack reuse' '
+	git config pack.allowPackReuse multi
+'
+
+test_expect_success 'reuse all objects from subset of bitmapped packs' '
+	test_commit C &&
+	git repack -d &&
+
+	git multi-pack-index write --bitmap &&
+
+	cat >in <<-EOF &&
+	$(git rev-parse C)
+	^$(git rev-parse A)
+	EOF
+
+	: >trace2.txt &&
+	GIT_TRACE2_EVENT="$PWD/trace2.txt" \
+		git pack-objects --stdout --revs <in >/dev/null &&
+
+	test_pack_reused 6 <trace2.txt &&
+	test_packs_reused 2 <trace2.txt
+'
+
+test_expect_success 'reuse all objects from all packs' '
+	: >trace2.txt &&
+	GIT_TRACE2_EVENT="$PWD/trace2.txt" \
+		git pack-objects --stdout --revs --all >/dev/null &&
+
+	test_pack_reused 9 <trace2.txt &&
+	test_packs_reused 3 <trace2.txt
+'
+
+test_expect_success 'reuse objects from first pack with middle gap' '
+	for i in D E F
+	do
+		test_commit "$i" || return 1
+	done &&
+
+	# Set "pack.window" to zero to ensure that we do not create any
+	# deltas, which could alter the amount of pack reuse we perform
+	# (if, for e.g., we are not sending one or more bases).
+	D="$(git -c pack.window=0 pack-objects --all --unpacked $packdir/pack)" &&
+
+	d_pos="$(pack_position $(git rev-parse D) <$packdir/pack-$D.idx)" &&
+	e_pos="$(pack_position $(git rev-parse E) <$packdir/pack-$D.idx)" &&
+	f_pos="$(pack_position $(git rev-parse F) <$packdir/pack-$D.idx)" &&
+
+	# commits F, E, and D, should appear in that order at the
+	# beginning of the pack
+	test $f_pos -lt $e_pos &&
+	test $e_pos -lt $d_pos &&
+
+	# Ensure that the pack we are constructing sorts ahead of any
+	# other packs in lexical/bitmap order by choosing it as the
+	# preferred pack.
+	git multi-pack-index write --bitmap --preferred-pack="pack-$D.idx" &&
+
+	cat >in <<-EOF &&
+	$(git rev-parse E)
+	^$(git rev-parse D)
+	EOF
+
+	: >trace2.txt &&
+	GIT_TRACE2_EVENT="$PWD/trace2.txt" \
+		git pack-objects --stdout --delta-base-offset --revs <in >/dev/null &&
+
+	test_pack_reused 3 <trace2.txt &&
+	test_packs_reused 1 <trace2.txt
+'
+
+test_expect_success 'reuse objects from middle pack with middle gap' '
+	rm -fr $packdir/multi-pack-index* &&
+
+	# Ensure that the pack we are constructing sort into any
+	# position *but* the first one, by choosing a different pack as
+	# the preferred one.
+	git multi-pack-index write --bitmap --preferred-pack="pack-$A.idx" &&
+
+	cat >in <<-EOF &&
+	$(git rev-parse E)
+	^$(git rev-parse D)
+	EOF
+
+	: >trace2.txt &&
+	GIT_TRACE2_EVENT="$PWD/trace2.txt" \
+		git pack-objects --stdout --delta-base-offset --revs <in >/dev/null &&
+
+	test_pack_reused 3 <trace2.txt &&
+	test_packs_reused 1 <trace2.txt
+'
+
+test_expect_success 'omit delta with uninteresting base (same pack)' '
+	git repack -adk &&
+
+	test_seq 32 >f &&
+	git add f &&
+	test_tick &&
+	git commit -m "delta" &&
+	delta="$(git rev-parse HEAD)" &&
+
+	test_seq 64 >f &&
+	test_tick &&
+	git commit -a -m "base" &&
+	base="$(git rev-parse HEAD)" &&
+
+	test_commit other &&
+
+	git repack -d &&
+
+	have_delta "$(git rev-parse $delta:f)" "$(git rev-parse $base:f)" &&
+
+	git multi-pack-index write --bitmap &&
+
+	cat >in <<-EOF &&
+	$(git rev-parse other)
+	^$base
+	EOF
+
+	: >trace2.txt &&
+	GIT_TRACE2_EVENT="$PWD/trace2.txt" \
+		git pack-objects --stdout --delta-base-offset --revs <in >/dev/null &&
+
+	# We can only reuse the 3 objects corresponding to "other" from
+	# the latest pack.
+	#
+	# This is because even though we want "delta", we do not want
+	# "base", meaning that we have to inflate the delta/base-pair
+	# corresponding to the blob in commit "delta", which bypasses
+	# the pack-reuse mechanism.
+	#
+	# The remaining objects from the other pack are similarly not
+	# reused because their objects are on the uninteresting side of
+	# the query.
+	test_pack_reused 3 <trace2.txt &&
+	test_packs_reused 1 <trace2.txt
+'
+
+test_expect_success 'omit delta from uninteresting base (cross pack)' '
+	cat >in <<-EOF &&
+	$(git rev-parse $base)
+	^$(git rev-parse $delta)
+	EOF
+
+	P="$(git pack-objects --revs $packdir/pack <in)" &&
+
+	git multi-pack-index write --bitmap --preferred-pack="pack-$P.idx" &&
+
+	: >trace2.txt &&
+	GIT_TRACE2_EVENT="$PWD/trace2.txt" \
+		git pack-objects --stdout --delta-base-offset --all >/dev/null &&
+
+	packs_nr="$(find $packdir -type f -name "pack-*.pack" | wc -l)" &&
+	objects_nr="$(git rev-list --count --all --objects)" &&
+
+	test_pack_reused $(($objects_nr - 1)) <trace2.txt &&
+	test_packs_reused $packs_nr <trace2.txt
+'
+
+test_done
--- a/t/t6113-rev-list-bitmap-filters.sh
+++ b/t/t6113-rev-list-bitmap-filters.sh
@ -4,6 +4,8 @@ test_description='rev-list combining bitmaps and filters'
 . ./test-lib.sh
 . "$TEST_DIRECTORY"/lib-bitmap.sh

+TEST_PASSES_SANITIZE_LEAK=true
+
 test_expect_success 'set up bitmapped repo' '
 	# one commit will have bitmaps, the other will not
 	test_commit one &&
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@ -1874,6 +1874,20 @@ test_region () {
 	return 0
 }

+# Check that the given data fragment was included as part of the
+# trace2-format trace on stdin.
+#
+#	test_trace2_data <category> <key> <value>
+#
+# For example, to look for trace2_data_intmax("pack-objects", repo,
+# "reused", N) in an invocation of "git pack-objects", run:
+#
+#	GIT_TRACE2_EVENT="$(pwd)/trace.txt" git pack-objects ... &&
+#	test_trace2_data pack-objects reused N <trace2.txt
+test_trace2_data () {
+	grep -e '"category":"'"$1"'","key":"'"$2"'","value":"'"$3"'"'
+}
+
 # Given a GIT_TRACE2_EVENT log over stdin, writes to stdout a list of URLs
 # sent to git-remote-https child processes.
 test_remote_https_urls() {