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

[Kaitao Cheng]

在 2026/10/29 02:47, Nhat Pham 写道:
> 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.
> 

This is only an accounting issue; I could not find any code in zswap that
reads lru->node[nid].nr_items.

>>
>> 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>

-- 
Thanks
Kaitao Cheng