[PATCH] zram: Avoid duplicate slots in batched writeback

[PATCH] zram: Avoid duplicate slots in batched writeback

[raoxu]

From: Xu Rao <raoxu@uniontech.com>

The writeback interface accepts multiple page_index and page_indexes
parameters in one operation. Their ranges can overlap, but
scan_slots_for_writeback() only excludes slots that are already being
written back or marked as same-filled.

A slot selected by an earlier range is placed on the post-processing list
and marked with ZRAM_PP_SLOT. Without checking that flag, an overlapping
range can enqueue the same slot again and submit it for writeback more
than once.

Skip slots already marked ZRAM_PP_SLOT while holding the slot lock. This
keeps each slot in a writeback scan session represented by at most one
post-processing entry and leaves the existing release and reuse paths
unchanged.

Signed-off-by: Xu Rao <raoxu@uniontech.com>
---
 drivers/block/zram/zram_drv.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index ab064c84b092..50a25989a23b 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -1215,6 +1215,9 @@ static void scan_slots_for_writeback(struct zram *zram, u32 mode,
 		if (!slot_allocated(zram, index))
 			goto next;

+		if (test_slot_flag(zram, index, ZRAM_PP_SLOT))
+			goto next;
+
 		if (test_slot_flag(zram, index, ZRAM_WB) ||
 		    test_slot_flag(zram, index, ZRAM_SAME))
 			goto next;
--
2.50.1

Re: [PATCH] zram: Avoid duplicate slots in batched writeback

[Sergey Senozhatsky]

On (26/11/10 14:18), raoxu wrote:
> The writeback interface accepts multiple page_index and page_indexes
> parameters in one operation. Their ranges can overlap, but
> scan_slots_for_writeback() only excludes slots that are already being
> written back or marked as same-filled.
> 
> A slot selected by an earlier range is placed on the post-processing list
> and marked with ZRAM_PP_SLOT. Without checking that flag, an overlapping
> range can enqueue the same slot again and submit it for writeback more
> than once.
> 
> Skip slots already marked ZRAM_PP_SLOT while holding the slot lock. This
> keeps each slot in a writeback scan session represented by at most one
> post-processing entry and leaves the existing release and reuse paths
> unchanged.
> 
> Signed-off-by: Xu Rao <raoxu@uniontech.com>

LGTM.
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>

Jens, do you want to pick it up and route via block tree or should
we route via "more traditional" tree (Andrew's tree)?