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

["David Hildenbrand (Red Hat)" <david@kernel.org>]

>>>
>>> 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.

> 
> But David also mentioned that in __read_swap_cache_async() there is a chance
> that anon folio in swapcache can have anon flag not set yet. +Chris and Kairui
> for more analysis.

Right, when we swapin an anon folio and did not map it into the page 
table yet. Likely we can trigger something similar when we proactively 
read a shmem page from swap into the swapcache.

So it's unclear "where" a swapcache page belongs to until we move it to 
its owner (anon / shmem), which is also why I cannot judge easily from

[   49.641309] migrating pfn b12f25 failed ret:7
[   49.641310] page: refcount:2 mapcount:0 mapping:0000000033bd8fe2 
index:0x7f404d925 pfn:0xb12f25
[   49.641311] aops:swap_aops
[   49.641313] flags: 
0x300000000030508(uptodate|active|owner_priv_1|reclaim|swapbacked|node=0|zone=3)
[   49.641314] raw: 0300000000030508 ffffed312c4bc908 ffffed312c4bc9c8 
0000000000000000
[   49.641315] raw: 00000007f404d925 00000000000c823b 00000002ffffffff 
0000000000000000
[   49.641315] page dumped because: migration failure

What exactly that was.

It was certainly an order-0 folio.

[...]

> 
> I agree with David. Acked-by: Zi Yan <ziy@nvidia.com>

Thanks for the fast review :)

-- 
Cheers

David