zram: support idle/huge page writeback

Add a new feature "zram idle/huge page writeback".  In the zram-swap use
case, zram usually has many idle/huge swap pages.  It's pointless to keep
them in memory (ie, zram).

To solve this problem, this feature introduces idle/huge page writeback to
the backing device so the goal is to save more memory space on embedded
systems.

Normal sequence to use idle/huge page writeback feature is as follows,

while (1) {
        # mark allocated zram slot to idle
        echo all > /sys/block/zram0/idle
        # leave system working for several hours
        # Unless there is no access for some blocks on zram,
	# they are still IDLE marked pages.

        echo "idle" > /sys/block/zram0/writeback
	or/and
	echo "huge" > /sys/block/zram0/writeback
        # write the IDLE or/and huge marked slot into backing device
	# and free the memory.
}

Per the discussion at
https://lore.kernel.org/lkml/20181122065926.GG3441@jagdpanzerIV/T/#u,

This patch removes direct incommpressibe page writeback feature
(d2afd25114f4 ("zram: write incompressible pages to backing device")).

Below concerns from Sergey:
== &< ==

"IDLE writeback" is superior to "incompressible writeback".

"incompressible writeback" is completely unpredictable and uncontrollable;
it depens on data patterns and compression algorithms.  While "IDLE
writeback" is predictable.

I even suspect, that, *ideally*, we can remove "incompressible writeback".
"IDLE pages" is a super set which also includes "incompressible" pages.
So, technically, we still can do "incompressible writeback" from "IDLE
writeback" path; but a much more reasonable one, based on a page idling
period.

I understand that you want to keep "direct incompressible writeback"
around.  ZRAM is especially popular on devices which do suffer from flash
wearout, so I can see "incompressible writeback" path becoming a dead
code, long term.

== &< ==

Below concerns from Minchan:
== &< ==

My concern is if we enable CONFIG_ZRAM_WRITEBACK in this implementation,
both hugepage/idlepage writeck will turn on.  However someuser want to
enable only idlepage writeback so we need to introduce turn on/off knob
for hugepage or new CONFIG_ZRAM_IDLEPAGE_WRITEBACK for those usecase.  I
don't want to make it complicated *if possible*.

Long term, I imagine we need to make VM aware of new swap hierarchy a
little bit different with as-is.  For example, first high priority swap
can return -EIO or -ENOCOMP, swap try to fallback to next lower priority
swap device.  With that, hugepage writeback will work tranparently.

So we could regard it as regression because incompressible pages doesn't
go to backing storage automatically.  Instead, user should do it via "echo
huge" > /sys/block/zram/writeback" manually.

== &< ==

Link: http://lkml.kernel.org/r/20181127055429.251614-6-minchan@kernel.org
Signed-off-by: Minchan Kim <minchan@kernel.org>
Reviewed-by: Joey Pabalinas <joeypabalinas@gmail.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/ABI/testing/sysfs-block-zram

@@ -106,3 +106,10 @@ Description:
 		idle file is write-only and mark zram slot as idle.
 		If system has mounted debugfs, user can see which slots
 		are idle via /sys/kernel/debug/zram/zram<id>/block_state
+
+What:		/sys/block/zram<id>/writeback
+Date:		November 2018
+Contact:	Minchan Kim <minchan@kernel.org>
+Description:
+		The writeback file is write-only and trigger idle and/or
+		huge page writeback to backing device.

Documentation/blockdev/zram.txt

@@ -238,11 +238,31 @@ line of text and contains the following stats separated by whitespace:
 
 = writeback
 
-With incompressible pages, there is no memory saving with zram.
-Instead, with CONFIG_ZRAM_WRITEBACK, zram can write incompressible page
+With CONFIG_ZRAM_WRITEBACK, zram can write idle/incompressible page
 to backing storage rather than keeping it in memory.
-User should set up backing device via /sys/block/zramX/backing_dev
-before disksize setting.
+To use the feature, admin should set up backing device via
+
+	"echo /dev/sda5 > /sys/block/zramX/backing_dev"
+
+before disksize setting. It supports only partition at this moment.
+If admin want to use incompressible page writeback, they could do via
+
+	"echo huge > /sys/block/zramX/write"
+
+To use idle page writeback, first, user need to declare zram pages
+as idle.
+
+	"echo all > /sys/block/zramX/idle"
+
+From now on, any pages on zram are idle pages. The idle mark
+will be removed until someone request access of the block.
+IOW, unless there is access request, those pages are still idle pages.
+
+Admin can request writeback of those idle pages at right timing via
+
+	"echo idle > /sys/block/zramX/writeback"
+
+With the command, zram writeback idle pages from memory to the storage.
 
 = memory tracking
 

drivers/block/zram/Kconfig

@@ -15,7 +15,7 @@ config ZRAM
 	  See Documentation/blockdev/zram.txt for more information.
 
 config ZRAM_WRITEBACK
-       bool "Write back incompressible page to backing device"
+       bool "Write back incompressible or idle page to backing device"
        depends on ZRAM
        help
 	 With incompressible page, there is no memory saving to keep it
@@ -23,6 +23,9 @@ config ZRAM_WRITEBACK
 	 For this feature, admin should set up backing device via
 	 /sys/block/zramX/backing_dev.
 
+	 With /sys/block/zramX/{idle,writeback}, application could ask
+	 idle page's writeback to the backing device to save in memory.
+
 	 See Documentation/blockdev/zram.txt for more information.
 
 config ZRAM_MEMORY_TRACKING

drivers/block/zram/zram_drv.c

@@ -52,6 +52,9 @@ static unsigned int num_devices = 1;
 static size_t huge_class_size;
 
 static void zram_free_page(struct zram *zram, size_t index);
+static int zram_bvec_read(struct zram *zram, struct bio_vec *bvec,
+				u32 index, int offset, struct bio *bio);
+
 
 static int zram_slot_trylock(struct zram *zram, u32 index)
 {
@@ -73,13 +76,6 @@ static inline bool init_done(struct zram *zram)
 	return zram->disksize;
 }
 
-static inline bool zram_allocated(struct zram *zram, u32 index)
-{
-
-	return (zram->table[index].flags >> (ZRAM_FLAG_SHIFT + 1)) ||
-					zram->table[index].handle;
-}
-
 static inline struct zram *dev_to_zram(struct device *dev)
 {
 	return (struct zram *)dev_to_disk(dev)->private_data;
@@ -138,6 +134,13 @@ static void zram_set_obj_size(struct zram *zram,
 	zram->table[index].flags = (flags << ZRAM_FLAG_SHIFT) | size;
 }
 
+static inline bool zram_allocated(struct zram *zram, u32 index)
+{
+	return zram_get_obj_size(zram, index) ||
+			zram_test_flag(zram, index, ZRAM_SAME) ||
+			zram_test_flag(zram, index, ZRAM_WB);
+}
+
 #if PAGE_SIZE != 4096
 static inline bool is_partial_io(struct bio_vec *bvec)
 {
@@ -308,10 +311,14 @@ static ssize_t idle_store(struct device *dev,
 	}
 
 	for (index = 0; index < nr_pages; index++) {
+		/*
+		 * Do not mark ZRAM_UNDER_WB slot as ZRAM_IDLE to close race.
+		 * See the comment in writeback_store.
+		 */
 		zram_slot_lock(zram, index);
-		if (!zram_allocated(zram, index))
+		if (!zram_allocated(zram, index) ||
+				zram_test_flag(zram, index, ZRAM_UNDER_WB))
 			goto next;
-
 		zram_set_flag(zram, index, ZRAM_IDLE);
 next:
 		zram_slot_unlock(zram, index);
@@ -546,6 +553,158 @@ static int read_from_bdev_async(struct zram *zram, struct bio_vec *bvec,
 	return 1;
 }
 
+#define HUGE_WRITEBACK 0x1
+#define IDLE_WRITEBACK 0x2
+
+static ssize_t writeback_store(struct device *dev,
+		struct device_attribute *attr, const char *buf, size_t len)
+{
+	struct zram *zram = dev_to_zram(dev);
+	unsigned long nr_pages = zram->disksize >> PAGE_SHIFT;
+	unsigned long index;
+	struct bio bio;
+	struct bio_vec bio_vec;
+	struct page *page;
+	ssize_t ret, sz;
+	char mode_buf[8];
+	unsigned long mode = -1UL;
+	unsigned long blk_idx = 0;
+
+	sz = strscpy(mode_buf, buf, sizeof(mode_buf));
+	if (sz <= 0)
+		return -EINVAL;
+
+	/* ignore trailing newline */
+	if (mode_buf[sz - 1] == '\n')
+		mode_buf[sz - 1] = 0x00;
+
+	if (!strcmp(mode_buf, "idle"))
+		mode = IDLE_WRITEBACK;
+	else if (!strcmp(mode_buf, "huge"))
+		mode = HUGE_WRITEBACK;
+
+	if (mode == -1UL)
+		return -EINVAL;
+
+	down_read(&zram->init_lock);
+	if (!init_done(zram)) {
+		ret = -EINVAL;
+		goto release_init_lock;
+	}
+
+	if (!zram->backing_dev) {
+		ret = -ENODEV;
+		goto release_init_lock;
+	}
+
+	page = alloc_page(GFP_KERNEL);
+	if (!page) {
+		ret = -ENOMEM;
+		goto release_init_lock;
+	}
+
+	for (index = 0; index < nr_pages; index++) {
+		struct bio_vec bvec;
+
+		bvec.bv_page = page;
+		bvec.bv_len = PAGE_SIZE;
+		bvec.bv_offset = 0;
+
+		if (!blk_idx) {
+			blk_idx = alloc_block_bdev(zram);
+			if (!blk_idx) {
+				ret = -ENOSPC;
+				break;
+			}
+		}
+
+		zram_slot_lock(zram, index);
+		if (!zram_allocated(zram, index))
+			goto next;
+
+		if (zram_test_flag(zram, index, ZRAM_WB) ||
+				zram_test_flag(zram, index, ZRAM_SAME) ||
+				zram_test_flag(zram, index, ZRAM_UNDER_WB))
+			goto next;
+
+		if ((mode & IDLE_WRITEBACK &&
+			  !zram_test_flag(zram, index, ZRAM_IDLE)) &&
+		    (mode & HUGE_WRITEBACK &&
+			  !zram_test_flag(zram, index, ZRAM_HUGE)))
+			goto next;
+		/*
+		 * Clearing ZRAM_UNDER_WB is duty of caller.
+		 * IOW, zram_free_page never clear it.
+		 */
+		zram_set_flag(zram, index, ZRAM_UNDER_WB);
+		/* Need for hugepage writeback racing */
+		zram_set_flag(zram, index, ZRAM_IDLE);
+		zram_slot_unlock(zram, index);
+		if (zram_bvec_read(zram, &bvec, index, 0, NULL)) {
+			zram_slot_lock(zram, index);
+			zram_clear_flag(zram, index, ZRAM_UNDER_WB);
+			zram_clear_flag(zram, index, ZRAM_IDLE);
+			zram_slot_unlock(zram, index);
+			continue;
+		}
+
+		bio_init(&bio, &bio_vec, 1);
+		bio_set_dev(&bio, zram->bdev);
+		bio.bi_iter.bi_sector = blk_idx * (PAGE_SIZE >> 9);
+		bio.bi_opf = REQ_OP_WRITE | REQ_SYNC;
+
+		bio_add_page(&bio, bvec.bv_page, bvec.bv_len,
+				bvec.bv_offset);
+		/*
+		 * XXX: A single page IO would be inefficient for write
+		 * but it would be not bad as starter.
+		 */
+		ret = submit_bio_wait(&bio);
+		if (ret) {
+			zram_slot_lock(zram, index);
+			zram_clear_flag(zram, index, ZRAM_UNDER_WB);
+			zram_clear_flag(zram, index, ZRAM_IDLE);
+			zram_slot_unlock(zram, index);
+			continue;
+		}
+
+		/*
+		 * We released zram_slot_lock so need to check if the slot was
+		 * changed. If there is freeing for the slot, we can catch it
+		 * easily by zram_allocated.
+		 * A subtle case is the slot is freed/reallocated/marked as
+		 * ZRAM_IDLE again. To close the race, idle_store doesn't
+		 * mark ZRAM_IDLE once it found the slot was ZRAM_UNDER_WB.
+		 * Thus, we could close the race by checking ZRAM_IDLE bit.
+		 */
+		zram_slot_lock(zram, index);
+		if (!zram_allocated(zram, index) ||
+			  !zram_test_flag(zram, index, ZRAM_IDLE)) {
+			zram_clear_flag(zram, index, ZRAM_UNDER_WB);
+			zram_clear_flag(zram, index, ZRAM_IDLE);
+			goto next;
+		}
+
+		zram_free_page(zram, index);
+		zram_clear_flag(zram, index, ZRAM_UNDER_WB);
+		zram_set_flag(zram, index, ZRAM_WB);
+		zram_set_element(zram, index, blk_idx);
+		blk_idx = 0;
+		atomic64_inc(&zram->stats.pages_stored);
+next:
+		zram_slot_unlock(zram, index);
+	}
+
+	if (blk_idx)
+		free_block_bdev(zram, blk_idx);
+	ret = len;
+	__free_page(page);
+release_init_lock:
+	up_read(&zram->init_lock);
+
+	return ret;
+}
+
 struct zram_work {
 	struct work_struct work;
 	struct zram *zram;
@@ -603,57 +762,8 @@ static int read_from_bdev(struct zram *zram, struct bio_vec *bvec,
 	else
 		return read_from_bdev_async(zram, bvec, entry, parent);
 }
-
-static int write_to_bdev(struct zram *zram, struct bio_vec *bvec,
-					u32 index, struct bio *parent,
-					unsigned long *pentry)
-{
-	struct bio *bio;
-	unsigned long entry;
-
-	bio = bio_alloc(GFP_ATOMIC, 1);
-	if (!bio)
-		return -ENOMEM;
-
-	entry = alloc_block_bdev(zram);
-	if (!entry) {
-		bio_put(bio);
-		return -ENOSPC;
-	}
-
-	bio->bi_iter.bi_sector = entry * (PAGE_SIZE >> 9);
-	bio_set_dev(bio, zram->bdev);
-	if (!bio_add_page(bio, bvec->bv_page, bvec->bv_len,
-					bvec->bv_offset)) {
-		bio_put(bio);
-		free_block_bdev(zram, entry);
-		return -EIO;
-	}
-
-	if (!parent) {
-		bio->bi_opf = REQ_OP_WRITE | REQ_SYNC;
-		bio->bi_end_io = zram_page_end_io;
-	} else {
-		bio->bi_opf = parent->bi_opf;
-		bio_chain(bio, parent);
-	}
-
-	submit_bio(bio);
-	*pentry = entry;
-
-	return 0;
-}
-
 #else
 static inline void reset_bdev(struct zram *zram) {};
-static int write_to_bdev(struct zram *zram, struct bio_vec *bvec,
-					u32 index, struct bio *parent,
-					unsigned long *pentry)
-
-{
-	return -EIO;
-}
-
 static int read_from_bdev(struct zram *zram, struct bio_vec *bvec,
 			unsigned long entry, struct bio *parent, bool sync)
 {
@@ -1006,7 +1116,8 @@ out:
 	atomic64_dec(&zram->stats.pages_stored);
 	zram_set_handle(zram, index, 0);
 	zram_set_obj_size(zram, index, 0);
-	WARN_ON_ONCE(zram->table[index].flags & ~(1UL << ZRAM_LOCK));
+	WARN_ON_ONCE(zram->table[index].flags &
+		~(1UL << ZRAM_LOCK | 1UL << ZRAM_UNDER_WB));
 }
 
 static int __zram_bvec_read(struct zram *zram, struct page *page, u32 index,
@@ -1115,7 +1226,6 @@ static int __zram_bvec_write(struct zram *zram, struct bio_vec *bvec,
 	struct page *page = bvec->bv_page;
 	unsigned long element = 0;
 	enum zram_pageflags flags = 0;
-	bool allow_wb = true;
 
 	mem = kmap_atomic(page);
 	if (page_same_filled(mem, &element)) {
@@ -1140,21 +1250,8 @@ compress_again:
 		return ret;
 	}
 
-	if (unlikely(comp_len >= huge_class_size)) {
+	if (comp_len >= huge_class_size)
 		comp_len = PAGE_SIZE;
-		if (zram->backing_dev && allow_wb) {
-			zcomp_stream_put(zram->comp);
-			ret = write_to_bdev(zram, bvec, index, bio, &element);
-			if (!ret) {
-				flags = ZRAM_WB;
-				ret = 1;
-				goto out;
-			}
-			allow_wb = false;
-			goto compress_again;
-		}
-	}
-
 	/*
 	 * handle allocation has 2 paths:
 	 * a) fast path is executed with preemption disabled (for
@@ -1643,6 +1740,7 @@ static DEVICE_ATTR_RW(max_comp_streams);
 static DEVICE_ATTR_RW(comp_algorithm);
 #ifdef CONFIG_ZRAM_WRITEBACK
 static DEVICE_ATTR_RW(backing_dev);
+static DEVICE_ATTR_WO(writeback);
 #endif
 
 static struct attribute *zram_disk_attrs[] = {
@@ -1657,6 +1755,7 @@ static struct attribute *zram_disk_attrs[] = {
 	&dev_attr_comp_algorithm.attr,
 #ifdef CONFIG_ZRAM_WRITEBACK
 	&dev_attr_backing_dev.attr,
+	&dev_attr_writeback.attr,
 #endif
 	&dev_attr_io_stat.attr,
 	&dev_attr_mm_stat.attr,

drivers/block/zram/zram_drv.h

@@ -47,6 +47,7 @@ enum zram_pageflags {
 	ZRAM_LOCK = ZRAM_FLAG_SHIFT,
 	ZRAM_SAME,	/* Page consists the same element */
 	ZRAM_WB,	/* page is stored on backing_device */
+	ZRAM_UNDER_WB,	/* page is under writeback */
 	ZRAM_HUGE,	/* Incompressible page */
 	ZRAM_IDLE,	/* not accessed page since last idle marking */