The patch titled Subject: zram: fix slot write race condition has been added to the -mm mm-hotfixes-unstable branch. Its filename is zram-fix-slot-write-race-condition.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Sergey Senozhatsky senozhatsky@chromium.org Subject: zram: fix slot write race condition Date: Tue, 9 Sep 2025 13:48:35 +0900

Parallel concurrent writes to the same zram index result in leaked zsmalloc handles. Schematically we can have something like this:

CPU0 CPU1 zram_slot_lock() zs_free(handle) zram_slot_lock() zram_slot_lock() zs_free(handle) zram_slot_lock()

compress compress handle = zs_malloc() handle = zs_malloc() zram_slot_lock zram_set_handle(handle) zram_slot_lock zram_slot_lock zram_set_handle(handle) zram_slot_lock

Either CPU0 or CPU1 zsmalloc handle will leak because zs_free() is done too early. In fact, we need to reset zram entry right before we set its new handle, all under the same slot lock scope.

Link: https://lkml.kernel.org/r/20250909045150.635345-1-senozhatsky@chromium.org Fixes: 71268035f5d73 ("zram: free slot memory early during write") Signed-off-by: Sergey Senozhatsky senozhatsky@chromium.org Reported-by: Changhui Zhong czhong@redhat.com Closes: https://lore.kernel.org/all/CAGVVp+UtpGoW5WEdEU7uVTtsSCjPN=ksN6EcvyypAtFDOUf... Cc: Jens Axboe axboe@kernel.dk Cc: Minchan Kim minchan@kernel.org Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

drivers/block/zram/zram_drv.c | 8 +++----- 1 file changed, 3 insertions(+), 5 deletions(-)

--- a/drivers/block/zram/zram_drv.c~zram-fix-slot-write-race-condition +++ a/drivers/block/zram/zram_drv.c @@ -1795,6 +1795,7 @@ static int write_same_filled_page(struct u32 index) { zram_slot_lock(zram, index); + zram_free_page(zram, index); zram_set_flag(zram, index, ZRAM_SAME); zram_set_handle(zram, index, fill); zram_slot_unlock(zram, index); @@ -1832,6 +1833,7 @@ static int write_incompressible_page(str kunmap_local(src);

zram_slot_lock(zram, index); + zram_free_page(zram, index); zram_set_flag(zram, index, ZRAM_HUGE); zram_set_handle(zram, index, handle); zram_set_obj_size(zram, index, PAGE_SIZE); @@ -1855,11 +1857,6 @@ static int zram_write_page(struct zram * unsigned long element; bool same_filled;

- /* First, free memory allocated to this slot (if any) */ - zram_slot_lock(zram, index); - zram_free_page(zram, index); - zram_slot_unlock(zram, index); - mem = kmap_local_page(page); same_filled = page_same_filled(mem, &element); kunmap_local(mem); @@ -1901,6 +1898,7 @@ static int zram_write_page(struct zram * zcomp_stream_put(zstrm);

zram_slot_lock(zram, index); + zram_free_page(zram, index); zram_set_handle(zram, index, handle); zram_set_obj_size(zram, index, comp_len); zram_slot_unlock(zram, index); _

Patches currently in -mm which might be from senozhatsky@chromium.org are

zram-fix-slot-write-race-condition.patch zram-protect-recomp_algorithm_show-with-init_lock.patch panic-remove-redundant-panic-cpu-backtrace.patch