Re: [PATCH] mm/zswap: fix double decrement of the LRU node count on writeback

[Nhat Pham]

On Wed, Oct 28, 2026 at 4:36 AM Kaitao Cheng <kaitao.cheng@xxxxxxxxx> wrote:
>
> From: Kaitao Cheng <chengkaitao@xxxxxxxxxx>
>
> Successful writeback calls zswap_entry_free(), which removes the entry
> from the LRU and decrements both the sublist and node item counts.
> shrink_memcg_cb() then returns LRU_REMOVED_RETRY, causing the LRU walker
> to decrement the node count again.
>
> The affected call path before this change is:
>
>   __list_lru_walk_one()
>     shrink_memcg_cb()
>       zswap_writeback_entry()
>         zswap_entry_free()
>           zswap_lru_del()
>             list_lru_del()
>               __list_lru_del()
>                 atomic_long_dec()  /* first node count decrement */
>       return LRU_REMOVED_RETRY
>     atomic_long_dec()              /* second node count decrement */
>
> Add zswap_lru_del_nocnt() to remove the entry and update only the sublist
> count, leaving node accounting to the walker. Split data cleanup into
> __zswap_entry_free() so writeback can free the entry without performing
> another LRU deletion. Other free paths retain their existing accounting.

What is the visible effect? Is there any effect, be it in reclaim
dynamics or userspace counters? Can you add it to the changelog?

I'm not against fixing it, but need to gauge the effect for
prioritization, whether to cc stable etc. etc.


>
> Fixes: 5878303c5353 ("mm/zswap: fix race between lru writeback and swapoff")
> Link: https://sashiko.dev/#/bug/linux-68d2ca49-23be-4504-9a75-b8526ec92388
> Signed-off-by: Kaitao Cheng <chengkaitao@xxxxxxxxxx>