[PATCH 0/2] zram: introduce compressed data writeback

[PATCH 0/2] zram: introduce compressed data writeback

[Sergey Senozhatsky]

As writeback becomes more common there is another shortcoming
that needs to be addressed - compressed data writeback.  Currently
zram does uncompressed data writeback which is not optimal due to
potential CPU and battery wastage.  This series changes suboptimal
uncompressed writeback to a more optimal compressed data writeback.

Richard Chang (1):
  zram: introduce compressed data writeback

Sergey Senozhatsky (1):
  zram: rename zram_free_page()

 drivers/block/zram/zram_drv.c | 260 ++++++++++++++++++++++++++--------
 1 file changed, 202 insertions(+), 58 deletions(-)

-- 
2.52.0.487.g5c8c507ade-goog

[PATCH 1/2] zram: introduce compressed data writeback

[Sergey Senozhatsky]

From: Richard Chang <richardycc@google.com>

zram stores all written back slots raw, which implies that
during writeback zram first has to decompress slots (except
for ZRAM_HUGE slots, which are raw already).  The problem
with this approach is that not every written back page gets
read back (either via read() or via page-fault), which means
that zram basically wastes CPU cycles and battery decompressing
such slots.  This changes with introduction of decompression
on demand, in other words decompression on read()/page-fault.

One caveat of decompression on demand is that async read
is completed in IRQ context, while zram decompression is
sleepable.  To workaround this, read-back decompression
is offloaded to a preemptible context - system high-prio
work-queue.

Signed-off-by: Richard Chang <richardycc@google.com>
Co-developed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Suggested-by: Minchan Kim <minchan@google.com>
Suggested-by: Brian Geffon <bgeffon@google.com>
---
 drivers/block/zram/zram_drv.c | 240 +++++++++++++++++++++++++++-------
 1 file changed, 192 insertions(+), 48 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 5759823d6314..eef6c0a675b5 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -57,8 +57,8 @@ static size_t huge_class_size;
 static const struct block_device_operations zram_devops;
 
 static void zram_free_page(struct zram *zram, size_t index);
-static int zram_read_from_zspool(struct zram *zram, struct page *page,
-				 u32 index);
+static int zram_read_from_zspool_raw(struct zram *zram, struct page *page,
+				     u32 index);
 
 #define slot_dep_map(zram, index) (&(zram)->table[(index)].dep_map)
 
@@ -522,6 +522,22 @@ struct zram_wb_req {
 	struct list_head entry;
 };
 
+struct zram_rb_req {
+	struct work_struct work;
+	struct zram *zram;
+	struct page *page;
+	/* The read bio for backing device */
+	struct bio *bio;
+	unsigned long blk_idx;
+	union {
+		/* The original bio to complete (async read) */
+		struct bio *parent;
+		/* error status (sync read) */
+		int error;
+	};
+	u32 index;
+};
+
 static ssize_t writeback_limit_enable_store(struct device *dev,
 					    struct device_attribute *attr,
 					    const char *buf, size_t len)
@@ -780,18 +796,6 @@ static void zram_release_bdev_block(struct zram *zram, unsigned long blk_idx)
 	atomic64_dec(&zram->stats.bd_count);
 }
 
-static void read_from_bdev_async(struct zram *zram, struct page *page,
-			unsigned long entry, struct bio *parent)
-{
-	struct bio *bio;
-
-	bio = bio_alloc(zram->bdev, 1, parent->bi_opf, GFP_NOIO);
-	bio->bi_iter.bi_sector = entry * (PAGE_SIZE >> 9);
-	__bio_add_page(bio, page, PAGE_SIZE, 0);
-	bio_chain(bio, parent);
-	submit_bio(bio);
-}
-
 static void release_wb_req(struct zram_wb_req *req)
 {
 	__free_page(req->page);
@@ -886,8 +890,9 @@ static void zram_account_writeback_submit(struct zram *zram)
 
 static int zram_writeback_complete(struct zram *zram, struct zram_wb_req *req)
 {
-	u32 index = req->pps->index;
-	int err;
+	u32 size, index = req->pps->index;
+	int err, prio;
+	bool huge;
 
 	err = blk_status_to_errno(req->bio.bi_status);
 	if (err) {
@@ -914,9 +919,22 @@ static int zram_writeback_complete(struct zram *zram, struct zram_wb_req *req)
 		goto out;
 	}
 
+	/*
+	 * ZRAM_WB slots get freed, we need to preserve data required for
+	 * read decompression.
+	 */
+	size = zram_get_obj_size(zram, index);
+	prio = zram_get_priority(zram, index);
+	huge = zram_test_flag(zram, index, ZRAM_HUGE);
+
 	zram_free_page(zram, index);
 	zram_set_flag(zram, index, ZRAM_WB);
+	if (huge)
+		zram_set_flag(zram, index, ZRAM_HUGE);
 	zram_set_handle(zram, index, req->blk_idx);
+	zram_set_obj_size(zram, index, size);
+	zram_set_priority(zram, index, prio);
+
 	atomic64_inc(&zram->stats.pages_stored);
 
 out:
@@ -1050,7 +1068,7 @@ static int zram_writeback_slots(struct zram *zram,
 		 */
 		if (!zram_test_flag(zram, index, ZRAM_PP_SLOT))
 			goto next;
-		if (zram_read_from_zspool(zram, req->page, index))
+		if (zram_read_from_zspool_raw(zram, req->page, index))
 			goto next;
 		zram_slot_unlock(zram, index);
 
@@ -1313,24 +1331,123 @@ static ssize_t writeback_store(struct device *dev,
 	return ret;
 }
 
-struct zram_work {
-	struct work_struct work;
-	struct zram *zram;
-	unsigned long entry;
-	struct page *page;
-	int error;
-};
+static int decompress_bdev_page(struct zram *zram, struct page *page, u32 index)
+{
+	struct zcomp_strm *zstrm;
+	unsigned int size;
+	int ret, prio;
+	void *src;
+
+	zram_slot_lock(zram, index);
+	/* Since slot was unlocked we need to make sure it's still ZRAM_WB */
+	if (!zram_test_flag(zram, index, ZRAM_WB)) {
+		zram_slot_unlock(zram, index);
+		/* We read some stale data, zero it out */
+		memset_page(page, 0, 0, PAGE_SIZE);
+		return -EIO;
+	}
+
+	if (zram_test_flag(zram, index, ZRAM_HUGE)) {
+		zram_slot_unlock(zram, index);
+		return 0;
+	}
+
+	size = zram_get_obj_size(zram, index);
+	prio = zram_get_priority(zram, index);
+
+	zstrm = zcomp_stream_get(zram->comps[prio]);
+	src = kmap_local_page(page);
+	ret = zcomp_decompress(zram->comps[prio], zstrm, src, size,
+			       zstrm->local_copy);
+	if (!ret)
+		copy_page(src, zstrm->local_copy);
+	kunmap_local(src);
+	zcomp_stream_put(zstrm);
+	zram_slot_unlock(zram, index);
+
+	return ret;
+}
+
+static void zram_deferred_decompress(struct work_struct *w)
+{
+	struct zram_rb_req *req = container_of(w, struct zram_rb_req, work);
+	struct page *page = bio_first_page_all(req->bio);
+	struct zram *zram = req->zram;
+	u32 index = req->index;
+	int ret;
+
+	ret = decompress_bdev_page(zram, page, index);
+	if (ret)
+		req->parent->bi_status = BLK_STS_IOERR;
+
+	/* Decrement parent's ->remaining */
+	bio_endio(req->parent);
+	bio_put(req->bio);
+	kfree(req);
+}
+
+static void zram_async_read_endio(struct bio *bio)
+{
+	struct zram_rb_req *req = bio->bi_private;
+
+	if (bio->bi_status) {
+		req->parent->bi_status = bio->bi_status;
+		bio_endio(req->parent);
+		bio_put(bio);
+		kfree(req);
+		return;
+	}
 
-static void zram_sync_read(struct work_struct *work)
+	/*
+	 * zram decompression is sleepable, so we need to deffer it to
+	 * a preemptible context.
+	 */
+	INIT_WORK(&req->work, zram_deferred_decompress);
+	queue_work(system_highpri_wq, &req->work);
+}
+
+static void read_from_bdev_async(struct zram *zram, struct page *page,
+				 u32 index, unsigned long blk_idx,
+				 struct bio *parent)
 {
-	struct zram_work *zw = container_of(work, struct zram_work, work);
+	struct zram_rb_req *req;
+	struct bio *bio;
+
+	req = kmalloc(sizeof(*req), GFP_NOIO);
+	if (!req)
+		return;
+
+	bio = bio_alloc(zram->bdev, 1, parent->bi_opf, GFP_NOIO);
+	if (!bio) {
+		kfree(req);
+		return;
+	}
+
+	req->zram = zram;
+	req->index = index;
+	req->blk_idx = blk_idx;
+	req->bio = bio;
+	req->parent = parent;
+
+	bio->bi_iter.bi_sector = blk_idx * (PAGE_SIZE >> 9);
+	bio->bi_private = req;
+	bio->bi_end_io = zram_async_read_endio;
+
+	__bio_add_page(bio, page, PAGE_SIZE, 0);
+	bio_inc_remaining(parent);
+	submit_bio(bio);
+}
+
+static void zram_sync_read(struct work_struct *w)
+{
+	struct zram_rb_req *req = container_of(w, struct zram_rb_req, work);
 	struct bio_vec bv;
 	struct bio bio;
 
-	bio_init(&bio, zw->zram->bdev, &bv, 1, REQ_OP_READ);
-	bio.bi_iter.bi_sector = zw->entry * (PAGE_SIZE >> 9);
-	__bio_add_page(&bio, zw->page, PAGE_SIZE, 0);
-	zw->error = submit_bio_wait(&bio);
+	bio_init(&bio, req->zram->bdev, &bv, 1, REQ_OP_READ);
+	bio.bi_iter.bi_sector = req->blk_idx * (PAGE_SIZE >> 9);
+	__bio_add_page(&bio, req->page, PAGE_SIZE, 0);
+	req->error = submit_bio_wait(&bio);
 }
 
 /*
@@ -1338,39 +1455,41 @@ static void zram_sync_read(struct work_struct *work)
  * chained IO with parent IO in same context, it's a deadlock. To avoid that,
  * use a worker thread context.
  */
-static int read_from_bdev_sync(struct zram *zram, struct page *page,
-				unsigned long entry)
+static int read_from_bdev_sync(struct zram *zram, struct page *page, u32 index,
+			       unsigned long blk_idx)
 {
-	struct zram_work work;
+	struct zram_rb_req req;
 
-	work.page = page;
-	work.zram = zram;
-	work.entry = entry;
+	req.page = page;
+	req.zram = zram;
+	req.blk_idx = blk_idx;
 
-	INIT_WORK_ONSTACK(&work.work, zram_sync_read);
-	queue_work(system_dfl_wq, &work.work);
-	flush_work(&work.work);
-	destroy_work_on_stack(&work.work);
+	INIT_WORK_ONSTACK(&req.work, zram_sync_read);
+	queue_work(system_dfl_wq, &req.work);
+	flush_work(&req.work);
+	destroy_work_on_stack(&req.work);
 
-	return work.error;
+	if (!req.error)
+		return decompress_bdev_page(zram, page, index);
+	return req.error;
 }
 
-static int read_from_bdev(struct zram *zram, struct page *page,
-			unsigned long entry, struct bio *parent)
+static int read_from_bdev(struct zram *zram, struct page *page, u32 index,
+			  unsigned long blk_idx, struct bio *parent)
 {
 	atomic64_inc(&zram->stats.bd_reads);
 	if (!parent) {
 		if (WARN_ON_ONCE(!IS_ENABLED(ZRAM_PARTIAL_IO)))
 			return -EIO;
-		return read_from_bdev_sync(zram, page, entry);
+		return read_from_bdev_sync(zram, page, index, blk_idx);
 	}
-	read_from_bdev_async(zram, page, entry, parent);
+	read_from_bdev_async(zram, page, index, blk_idx, parent);
 	return 0;
 }
 #else
 static inline void reset_bdev(struct zram *zram) {};
-static int read_from_bdev(struct zram *zram, struct page *page,
-			unsigned long entry, struct bio *parent)
+static int read_from_bdev(struct zram *zram, struct page *page, u32 index,
+			  unsigned long blk_idx, struct bio *parent)
 {
 	return -EIO;
 }
@@ -1977,6 +2096,31 @@ static int read_compressed_page(struct zram *zram, struct page *page, u32 index)
 	return ret;
 }
 
+static int zram_read_from_zspool_raw(struct zram *zram, struct page *page,
+				     u32 index)
+{
+	struct zcomp_strm *zstrm;
+	unsigned long handle;
+	unsigned int size;
+	void *src;
+
+	handle = zram_get_handle(zram, index);
+	size = zram_get_obj_size(zram, index);
+
+	/*
+	 * We need to get stream just for ->local_copy buffer, in
+	 * case if object spans two physical pages. No decompression
+	 * takes place here, as we read raw compressed data.
+	 */
+	zstrm = zcomp_stream_get(zram->comps[ZRAM_PRIMARY_COMP]);
+	src = zs_obj_read_begin(zram->mem_pool, handle, zstrm->local_copy);
+	memcpy_to_page(page, 0, src, size);
+	zs_obj_read_end(zram->mem_pool, handle, src);
+	zcomp_stream_put(zstrm);
+
+	return 0;
+}
+
 /*
  * Reads (decompresses if needed) a page from zspool (zsmalloc).
  * Corresponding ZRAM slot should be locked.
@@ -2012,7 +2156,7 @@ static int zram_read_page(struct zram *zram, struct page *page, u32 index,
 		 * device.
 		 */
 		zram_slot_unlock(zram, index);
-		ret = read_from_bdev(zram, page, blk_idx, parent);
+		ret = read_from_bdev(zram, page, index, blk_idx, parent);
 	}
 
 	/* Should NEVER happen. Return bio error if it does. */
-- 
2.52.0.487.g5c8c507ade-goog

[PATCH 2/2] zram: rename zram_free_page()

[Sergey Senozhatsky]

We don't free page in zram_free_page(), not all slots even
have any memory associated with them (e.g. ZRAM_SAME).  We
free the slot (or reset it), rename the function accordingly.

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
---
 drivers/block/zram/zram_drv.c | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index eef6c0a675b5..d8054b3cafaa 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -56,7 +56,7 @@ static size_t huge_class_size;
 
 static const struct block_device_operations zram_devops;
 
-static void zram_free_page(struct zram *zram, size_t index);
+static void zram_slot_free(struct zram *zram, u32 index);
 static int zram_read_from_zspool_raw(struct zram *zram, struct page *page,
 				     u32 index);
 
@@ -927,7 +927,7 @@ static int zram_writeback_complete(struct zram *zram, struct zram_wb_req *req)
 	prio = zram_get_priority(zram, index);
 	huge = zram_test_flag(zram, index, ZRAM_HUGE);
 
-	zram_free_page(zram, index);
+	zram_slot_free(zram, index);
 	zram_set_flag(zram, index, ZRAM_WB);
 	if (huge)
 		zram_set_flag(zram, index, ZRAM_HUGE);
@@ -1966,7 +1966,7 @@ static void zram_meta_free(struct zram *zram, u64 disksize)
 
 	/* Free all pages that are still in this zram device */
 	for (index = 0; index < num_pages; index++)
-		zram_free_page(zram, index);
+		zram_slot_free(zram, index);
 
 	zs_destroy_pool(zram->mem_pool);
 	vfree(zram->table);
@@ -1998,7 +1998,7 @@ static bool zram_meta_alloc(struct zram *zram, u64 disksize)
 	return true;
 }
 
-static void zram_free_page(struct zram *zram, size_t index)
+static void zram_slot_free(struct zram *zram, u32 index)
 {
 	unsigned long handle;
 
@@ -2197,7 +2197,7 @@ static int write_same_filled_page(struct zram *zram, unsigned long fill,
 				  u32 index)
 {
 	zram_slot_lock(zram, index);
-	zram_free_page(zram, index);
+	zram_slot_free(zram, index);
 	zram_set_flag(zram, index, ZRAM_SAME);
 	zram_set_handle(zram, index, fill);
 	zram_slot_unlock(zram, index);
@@ -2235,7 +2235,7 @@ static int write_incompressible_page(struct zram *zram, struct page *page,
 	kunmap_local(src);
 
 	zram_slot_lock(zram, index);
-	zram_free_page(zram, index);
+	zram_slot_free(zram, index);
 	zram_set_flag(zram, index, ZRAM_HUGE);
 	zram_set_handle(zram, index, handle);
 	zram_set_obj_size(zram, index, PAGE_SIZE);
@@ -2300,7 +2300,7 @@ static int zram_write_page(struct zram *zram, struct page *page, u32 index)
 	zcomp_stream_put(zstrm);
 
 	zram_slot_lock(zram, index);
-	zram_free_page(zram, index);
+	zram_slot_free(zram, index);
 	zram_set_handle(zram, index, handle);
 	zram_set_obj_size(zram, index, comp_len);
 	zram_slot_unlock(zram, index);
@@ -2522,7 +2522,7 @@ static int recompress_slot(struct zram *zram, u32 index, struct page *page,
 	zs_obj_write(zram->mem_pool, handle_new, zstrm->buffer, comp_len_new);
 	zcomp_stream_put(zstrm);
 
-	zram_free_page(zram, index);
+	zram_slot_free(zram, index);
 	zram_set_handle(zram, index, handle_new);
 	zram_set_obj_size(zram, index, comp_len_new);
 	zram_set_priority(zram, index, prio);
@@ -2725,7 +2725,7 @@ static void zram_bio_discard(struct zram *zram, struct bio *bio)
 
 	while (n >= PAGE_SIZE) {
 		zram_slot_lock(zram, index);
-		zram_free_page(zram, index);
+		zram_slot_free(zram, index);
 		zram_slot_unlock(zram, index);
 		atomic64_inc(&zram->stats.notify_free);
 		index++;
@@ -2833,7 +2833,7 @@ static void zram_slot_free_notify(struct block_device *bdev,
 		return;
 	}
 
-	zram_free_page(zram, index);
+	zram_slot_free(zram, index);
 	zram_slot_unlock(zram, index);
 }
 
-- 
2.52.0.487.g5c8c507ade-goog

Re: [PATCH 1/2] zram: introduce compressed data writeback

[kernel test robot]

Hi Sergey,

kernel test robot noticed the following build warnings:

[auto build test WARNING on akpm-mm/mm-everything]
[also build test WARNING on next-20251128]
[cannot apply to axboe/for-next linus/master v6.18-rc7]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Sergey-Senozhatsky/zram-introduce-compressed-data-writeback/20251129-010716
base:   https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
patch link:    https://lore.kernel.org/r/20251128170442.2988502-2-senozhatsky%40chromium.org
patch subject: [PATCH 1/2] zram: introduce compressed data writeback
config: x86_64-randconfig-011-20251129 (https://download.01.org/0day-ci/archive/20251129/202511291628.NZif1jdx-lkp@intel.com/config)
compiler: gcc-14 (Debian 14.2.0-19) 14.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20251129/202511291628.NZif1jdx-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202511291628.NZif1jdx-lkp@intel.com/

All warnings (new ones prefixed by >>):

>> drivers/block/zram/zram_drv.c:2099:12: warning: 'zram_read_from_zspool_raw' defined but not used [-Wunused-function]
    2099 | static int zram_read_from_zspool_raw(struct zram *zram, struct page *page,
         |            ^~~~~~~~~~~~~~~~~~~~~~~~~


vim +/zram_read_from_zspool_raw +2099 drivers/block/zram/zram_drv.c

  2098	
> 2099	static int zram_read_from_zspool_raw(struct zram *zram, struct page *page,
  2100					     u32 index)
  2101	{
  2102		struct zcomp_strm *zstrm;
  2103		unsigned long handle;
  2104		unsigned int size;
  2105		void *src;
  2106	
  2107		handle = zram_get_handle(zram, index);
  2108		size = zram_get_obj_size(zram, index);
  2109	
  2110		/*
  2111		 * We need to get stream just for ->local_copy buffer, in
  2112		 * case if object spans two physical pages. No decompression
  2113		 * takes place here, as we read raw compressed data.
  2114		 */
  2115		zstrm = zcomp_stream_get(zram->comps[ZRAM_PRIMARY_COMP]);
  2116		src = zs_obj_read_begin(zram->mem_pool, handle, zstrm->local_copy);
  2117		memcpy_to_page(page, 0, src, size);
  2118		zs_obj_read_end(zram->mem_pool, handle, src);
  2119		zcomp_stream_put(zstrm);
  2120	
  2121		return 0;
  2122	}
  2123	

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH 1/2] zram: introduce compressed data writeback

[Barry Song]

On Sat, Nov 29, 2025 at 1:06 AM Sergey Senozhatsky
<senozhatsky@chromium.org> wrote:
>
> From: Richard Chang <richardycc@google.com>
>

Hi Richard, Sergey,

Thanks a lot for developing this. For years, people have been looking for
compressed data writeback to reduce I/O, such as compacting multiple compressed
blocks into a single page on block devices. I guess this patchset hasn’t reached
that point yet, right?

> zram stores all written back slots raw, which implies that
> during writeback zram first has to decompress slots (except
> for ZRAM_HUGE slots, which are raw already).  The problem
> with this approach is that not every written back page gets
> read back (either via read() or via page-fault), which means
> that zram basically wastes CPU cycles and battery decompressing
> such slots.  This changes with introduction of decompression

If a page is swapped out and never read again, does that actually indicate
a memory leak in userspace?

So the main benefit of this patch so far is actually avoiding decompression
for "leaked" anon pages, which might still have a pointer but are
never accessed again?

> on demand, in other words decompression on read()/page-fault.

Thanks
Barry

Re: [PATCH 1/2] zram: introduce compressed data writeback

[Sergey Senozhatsky]

Hi Barry,

On (25/11/29 17:55), Barry Song wrote:
> On Sat, Nov 29, 2025 at 1:06 AM Sergey Senozhatsky
> <senozhatsky@chromium.org> wrote:
> >
> > From: Richard Chang <richardycc@google.com>
> >
> 
> Hi Richard, Sergey,
> 
> Thanks a lot for developing this. For years, people have been looking for
> compressed data writeback to reduce I/O, such as compacting multiple compressed
> blocks into a single page on block devices. I guess this patchset hasn’t reached
> that point yet, right?

Right.

> > zram stores all written back slots raw, which implies that
> > during writeback zram first has to decompress slots (except
> > for ZRAM_HUGE slots, which are raw already).  The problem
> > with this approach is that not every written back page gets
> > read back (either via read() or via page-fault), which means
> > that zram basically wastes CPU cycles and battery decompressing
> > such slots.  This changes with introduction of decompression
> 
> If a page is swapped out and never read again, does that actually indicate
> a memory leak in userspace?

No, it just means that there is no page-fault on that page.  E.g. we
swapped out an unused browser tab and never come back to it within the
session: e.g. user closed the tab/app, or logged out of session, or
rebooted the device, or simply powered off (desktop/laptop).

Re: [PATCH 0/2] zram: introduce compressed data writeback

[Sergey Senozhatsky]

On (25/11/29 02:04), Sergey Senozhatsky wrote:
> As writeback becomes more common there is another shortcoming
> that needs to be addressed - compressed data writeback.  Currently
> zram does uncompressed data writeback which is not optimal due to
> potential CPU and battery wastage.  This series changes suboptimal
> uncompressed writeback to a more optimal compressed data writeback.

JFI, v2 is coming.

Re: [PATCH 1/2] zram: introduce compressed data writeback

[Barry Song]

On Mon, Dec 1, 2025 at 11:56 AM Sergey Senozhatsky
<senozhatsky@chromium.org> wrote:
[...]
> > > zram stores all written back slots raw, which implies that
> > > during writeback zram first has to decompress slots (except
> > > for ZRAM_HUGE slots, which are raw already).  The problem
> > > with this approach is that not every written back page gets
> > > read back (either via read() or via page-fault), which means
> > > that zram basically wastes CPU cycles and battery decompressing
> > > such slots.  This changes with introduction of decompression
> >
> > If a page is swapped out and never read again, does that actually indicate
> > a memory leak in userspace?
>
> No, it just means that there is no page-fault on that page.  E.g. we
> swapped out an unused browser tab and never come back to it within the
> session: e.g. user closed the tab/app, or logged out of session, or
> rebooted the device, or simply powered off (desktop/laptop).

Thanks, Sergey. That makes sense to me. On Android, users don’t have a
close button, yet apps can still be OOM-killed; those pages are never
swapped in.

Thanks
Barry

Re: [PATCH 1/2] zram: introduce compressed data writeback

[Sergey Senozhatsky]

On (25/12/01 16:59), Barry Song wrote:
> On Mon, Dec 1, 2025 at 11:56 AM Sergey Senozhatsky
> <senozhatsky@chromium.org> wrote:
> [...]
> > > > zram stores all written back slots raw, which implies that
> > > > during writeback zram first has to decompress slots (except
> > > > for ZRAM_HUGE slots, which are raw already).  The problem
> > > > with this approach is that not every written back page gets
> > > > read back (either via read() or via page-fault), which means
> > > > that zram basically wastes CPU cycles and battery decompressing
> > > > such slots.  This changes with introduction of decompression
> > >
> > > If a page is swapped out and never read again, does that actually indicate
> > > a memory leak in userspace?
> >
> > No, it just means that there is no page-fault on that page.  E.g. we
> > swapped out an unused browser tab and never come back to it within the
> > session: e.g. user closed the tab/app, or logged out of session, or
> > rebooted the device, or simply powered off (desktop/laptop).
> 
> Thanks, Sergey. That makes sense to me. On Android, users don’t have a
> close button, yet apps can still be OOM-killed; those pages are never
> swapped in.

I see.  I suppose on android you still can swipe up and terminate
un-needed apps, wouldn't this be the same?  Well, apart from that,
zram is not android-specific, some distros use it on desktops/laptops
as well.

Re: [PATCH 1/2] zram: introduce compressed data writeback

[Sergey Senozhatsky]

On (25/11/29 17:07), kernel test robot wrote:
> Hi Sergey,
> 
> kernel test robot noticed the following build warnings:
> 
> [auto build test WARNING on akpm-mm/mm-everything]
> [also build test WARNING on next-20251128]
> [cannot apply to axboe/for-next linus/master v6.18-rc7]
> [If your patch is applied to the wrong git tree, kindly drop us a note.
> And when submitting patch, we suggest to use '--base' as documented in
> https://git-scm.com/docs/git-format-patch#_base_tree_information]
> 
> url:    https://github.com/intel-lab-lkp/linux/commits/Sergey-Senozhatsky/zram-introduce-compressed-data-writeback/20251129-010716
> base:   https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
> patch link:    https://lore.kernel.org/r/20251128170442.2988502-2-senozhatsky%40chromium.org
> patch subject: [PATCH 1/2] zram: introduce compressed data writeback
> config: x86_64-randconfig-011-20251129 (https://download.01.org/0day-ci/archive/20251129/202511291628.NZif1jdx-lkp@intel.com/config)
> compiler: gcc-14 (Debian 14.2.0-19) 14.2.0
> reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20251129/202511291628.NZif1jdx-lkp@intel.com/reproduce)
> 
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <lkp@intel.com>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202511291628.NZif1jdx-lkp@intel.com/
> 
> All warnings (new ones prefixed by >>):
> 
> >> drivers/block/zram/zram_drv.c:2099:12: warning: 'zram_read_from_zspool_raw' defined but not used [-Wunused-function]
>     2099 | static int zram_read_from_zspool_raw(struct zram *zram, struct page *page,
>          |            ^~~~~~~~~~~~~~~~~~~~~~~~~

Fixed in v2.  Thanks.

Re: [PATCH 1/2] zram: introduce compressed data writeback

[Barry Song]

On Mon, Dec 1, 2025 at 5:09 PM Sergey Senozhatsky
<senozhatsky@chromium.org> wrote:
>
> On (25/12/01 16:59), Barry Song wrote:
> > On Mon, Dec 1, 2025 at 11:56 AM Sergey Senozhatsky
> > <senozhatsky@chromium.org> wrote:
> > [...]
> > > > > zram stores all written back slots raw, which implies that
> > > > > during writeback zram first has to decompress slots (except
> > > > > for ZRAM_HUGE slots, which are raw already).  The problem
> > > > > with this approach is that not every written back page gets
> > > > > read back (either via read() or via page-fault), which means
> > > > > that zram basically wastes CPU cycles and battery decompressing
> > > > > such slots.  This changes with introduction of decompression
> > > >
> > > > If a page is swapped out and never read again, does that actually indicate
> > > > a memory leak in userspace?
> > >
> > > No, it just means that there is no page-fault on that page.  E.g. we
> > > swapped out an unused browser tab and never come back to it within the
> > > session: e.g. user closed the tab/app, or logged out of session, or
> > > rebooted the device, or simply powered off (desktop/laptop).
> >
> > Thanks, Sergey. That makes sense to me. On Android, users don’t have a
> > close button, yet apps can still be OOM-killed; those pages are never
> > swapped in.
>
> I see.  I suppose on android you still can swipe up and terminate
> un-needed apps, wouldn't this be the same?  Well, apart from that,

That’s true, although it’s not typical user behavior :-)

> zram is not android-specific, some distros use it on desktops/laptops
> as well.

Yes, absolutely.