Refactor packed_git to prepare for sliding mmap windows.

The idea behind the sliding mmap window pack reader implementation
is to have multiple mmap regions active against the same pack file,
thereby allowing the process to mmap in only the active/hot sections
of the pack and reduce overall virtual address space usage.

To implement this we need to refactor the mmap related data
(pack_base, pack_use_cnt) out of struct packed_git and move them
into a new struct pack_window.

We are refactoring the code to support a single struct pack_window
per packfile, thereby emulating the prior behavior of mmap'ing the
entire pack file.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-pack-objects.c

@@ -438,7 +438,7 @@ static unsigned long write_object(struct sha1file *f,
 		}
 
 		use_packed_git(p);
-		buf = (char *) p->pack_base
+		buf = p->windows->base
 			+ entry->in_pack_offset
 			+ entry->in_pack_header_size;
 		datalen = find_packed_object_size(p, entry->in_pack_offset)
@@ -943,7 +943,7 @@ static void check_object(struct object_entry *entry)
 		struct object_entry *base_entry = NULL;
 
 		use_packed_git(p);
-		buf = p->pack_base;
+		buf = p->windows->base;
 		buf += entry->in_pack_offset;
 
 		/* We want in_pack_type even if we do not reuse delta.

cache.h

@@ -336,14 +336,21 @@ extern struct alternate_object_database {
 } *alt_odb_list;
 extern void prepare_alt_odb(void);
 
+struct pack_window {
+	struct pack_window *next;
+	unsigned char *base;
+	off_t offset;
+	size_t len;
+	unsigned int last_used;
+	unsigned int inuse_cnt;
+};
+
 extern struct packed_git {
 	struct packed_git *next;
 	unsigned long index_size;
 	unsigned long pack_size;
+	struct pack_window *windows;
 	unsigned int *index_base;
-	void *pack_base;
-	unsigned int pack_last_used;
-	unsigned int pack_use_cnt;
 	int pack_local;
 	unsigned char sha1[20];
 	/* something like ".git/objects/pack/xxxxx.pack" */

pack-check.c

@@ -13,7 +13,8 @@ static int verify_packfile(struct packed_git *p)
 	int nr_objects, err, i;
 
 	/* Header consistency check */
-	hdr = p->pack_base;
+	pack_base = p->windows->base;
+	hdr = (struct pack_header*)pack_base;
 	if (hdr->hdr_signature != htonl(PACK_SIGNATURE))
 		return error("Packfile %s signature mismatch", p->pack_name);
 	if (!pack_version_ok(hdr->hdr_version))
@@ -26,7 +27,6 @@ static int verify_packfile(struct packed_git *p)
 			     num_packed_objects(p));
 
 	SHA1_Init(&ctx);
-	pack_base = p->pack_base;
 	SHA1_Update(&ctx, pack_base, pack_size - 20);
 	SHA1_Final(sha1, &ctx);
 	if (hashcmp(sha1, (unsigned char *)pack_base + pack_size - 20))
@@ -78,7 +78,7 @@ static void show_pack_info(struct packed_git *p)
 	int nr_objects, i;
 	unsigned int chain_histogram[MAX_CHAIN];
 
-	hdr = p->pack_base;
+	hdr = (struct pack_header*)p->windows->base;
 	nr_objects = ntohl(hdr->hdr_entries);
 	memset(chain_histogram, 0, sizeof(chain_histogram));
 

sha1_file.c

@@ -455,21 +455,23 @@ static int unuse_one_packed_git(void)
 	struct packed_git *p, *lru = NULL;
 
 	for (p = packed_git; p; p = p->next) {
-		if (p->pack_use_cnt || !p->pack_base)
+		if (!p->windows || p->windows->inuse_cnt)
 			continue;
-		if (!lru || p->pack_last_used < lru->pack_last_used)
+		if (!lru || p->windows->last_used < lru->windows->last_used)
 			lru = p;
 	}
 	if (!lru)
 		return 0;
-	munmap(lru->pack_base, lru->pack_size);
-	lru->pack_base = NULL;
+	munmap(lru->windows->base, lru->windows->len);
+	free(lru->windows);
+	lru->windows = NULL;
 	return 1;
 }
 
 void unuse_packed_git(struct packed_git *p)
 {
-	p->pack_use_cnt--;
+	if (p->windows)
+		p->windows->inuse_cnt--;
 }
 
 int use_packed_git(struct packed_git *p)
@@ -482,10 +484,10 @@ int use_packed_git(struct packed_git *p)
 			die("packfile %s not a regular file", p->pack_name);
 		p->pack_size = st.st_size;
 	}
-	if (!p->pack_base) {
+	if (!p->windows) {
 		int fd;
 		struct stat st;
-		void *map;
+		struct pack_window *win;
 		struct pack_header *hdr;
 
 		pack_mapped += p->pack_size;
@@ -500,16 +502,18 @@ int use_packed_git(struct packed_git *p)
 		}
 		if (st.st_size != p->pack_size)
 			die("packfile %s size mismatch.", p->pack_name);
-		map = mmap(NULL, p->pack_size, PROT_READ, MAP_PRIVATE, fd, 0);
+		win = xcalloc(1, sizeof(*win));
+		win->len = p->pack_size;
+		win->base = mmap(NULL, p->pack_size, PROT_READ, MAP_PRIVATE, fd, 0);
 		close(fd);
-		if (map == MAP_FAILED)
+		if (win->base == MAP_FAILED)
 			die("packfile %s cannot be mapped.", p->pack_name);
-		p->pack_base = map;
+		p->windows = win;
 
 		/* Check if we understand this pack file.  If we don't we're
 		 * likely too old to handle it.
 		 */
-		hdr = map;
+		hdr = (struct pack_header*)win->base;
 		if (hdr->hdr_signature != htonl(PACK_SIGNATURE))
 			die("packfile %s isn't actually a pack.", p->pack_name);
 		if (!pack_version_ok(hdr->hdr_version))
@@ -522,13 +526,13 @@ int use_packed_git(struct packed_git *p)
 		 */
 		if (hashcmp((unsigned char *)(p->index_base) +
 			    p->index_size - 40,
-			    (unsigned char *)p->pack_base +
+			    p->windows->base +
 			    p->pack_size - 20)) {
 			die("packfile %s does not match index.", p->pack_name);
 		}
 	}
-	p->pack_last_used = pack_used_ctr++;
-	p->pack_use_cnt++;
+	p->windows->last_used = pack_used_ctr++;
+	p->windows->inuse_cnt++;
 	return 0;
 }
 
@@ -558,9 +562,7 @@ struct packed_git *add_packed_git(char *path, int path_len, int local)
 	p->pack_size = st.st_size;
 	p->index_base = idx_map;
 	p->next = NULL;
-	p->pack_base = NULL;
-	p->pack_last_used = 0;
-	p->pack_use_cnt = 0;
+	p->windows = NULL;
 	p->pack_local = local;
 	if ((path_len > 44) && !get_sha1_hex(path + path_len - 44, sha1))
 		hashcpy(p->sha1, sha1);
@@ -591,9 +593,7 @@ struct packed_git *parse_pack_index_file(const unsigned char *sha1, char *idx_pa
 	p->pack_size = 0;
 	p->index_base = idx_map;
 	p->next = NULL;
-	p->pack_base = NULL;
-	p->pack_last_used = 0;
-	p->pack_use_cnt = 0;
+	p->windows = NULL;
 	hashcpy(p->sha1, sha1);
 	return p;
 }
@@ -882,7 +882,7 @@ static unsigned long get_delta_base(struct packed_git *p,
 				    unsigned long delta_obj_offset,
 				    unsigned long *base_obj_offset)
 {
-	unsigned char *base_info = (unsigned char *) p->pack_base + offset;
+	unsigned char *base_info = p->windows->base + offset;
 	unsigned long base_offset;
 
 	/* there must be at least 20 bytes left regardless of delta type */
@@ -949,7 +949,7 @@ static int packed_delta_info(struct packed_git *p,
 
 		memset(&stream, 0, sizeof(stream));
 
-		stream.next_in = (unsigned char *) p->pack_base + offset;
+		stream.next_in = p->windows->base + offset;
 		stream.avail_in = p->pack_size - offset;
 		stream.next_out = delta_head;
 		stream.avail_out = sizeof(delta_head);
@@ -984,8 +984,7 @@ static unsigned long unpack_object_header(struct packed_git *p, unsigned long of
 	if (p->pack_size <= offset)
 		die("object offset outside of pack file");
 
-	used = unpack_object_header_gently((unsigned char *)p->pack_base +
-					   offset,
+	used = unpack_object_header_gently(p->windows->base + offset,
 					   p->pack_size - offset, type, sizep);
 	if (!used)
 		die("object offset outside of pack file");
@@ -1031,7 +1030,7 @@ void packed_object_info_detail(struct packed_git *p,
 			if (p->pack_size <= offset + 20)
 				die("pack file %s records an incomplete delta base",
 				    p->pack_name);
-			next_sha1 = (unsigned char *) p->pack_base + offset;
+			next_sha1 = p->windows->base + offset;
 			if (*delta_chain_length == 0)
 				hashcpy(base_sha1, next_sha1);
 			offset = find_pack_entry_one(next_sha1, p);
@@ -1081,7 +1080,7 @@ static void *unpack_compressed_entry(struct packed_git *p,
 	buffer = xmalloc(size + 1);
 	buffer[size] = 0;
 	memset(&stream, 0, sizeof(stream));
-	stream.next_in = (unsigned char*)p->pack_base + offset;
+	stream.next_in = p->windows->base + offset;
 	stream.avail_in = p->pack_size - offset;
 	stream.next_out = buffer;
 	stream.avail_out = size;