Re: [PATCH] mm: Consider non-anon swap cache folios in folio_expected_ref_count()

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2025/12/19 08:21, Wei Yang wrote:
> On Wed, Dec 17, 2025 at 02:04:16AM +0100, David Hildenbrand (Red Hat) wrote:
>>>>>
>>>>> I am not very familiar with the memory hot-(un)plug or swapping code, so
>>>>> I am not 100% certain if this patch actually solves the root of the
>>>>> problem. I believe the issue is from shmem folios, in which case I believe
>>>>> this patch is correct. However, I couldn't think of an easy way to confirm
>>>>> that the affected folios were from shmem. I guess it could be possible that
>>>>> the root cause could be from some bug where some anonymous pages do not
>>>>> return true to folio_test_anon(). I don't think that's the case, but
>>>>> figured the MM maintainers would have a better idea of what's going on.
>>>
>>> I am not sure about if shmem in swapcache causes the issue, since
>>> the above setup does not involve shmem. +Baolin and Hugh for some insight.
>>
>> We might just push out another unrelated shmem page to swap as we create
>> memory pressure in the system I think.
>>
> 
> One trivial question: currently we only put anon/shmem folio in swapcache,
> right?

AFAICT, Yes (note a special case for anonymous folios: lazyfree 
anonymous folios will be directly freed instead of being swapped out).