Re: [PATCH V3 1/2] mm/khugepaged: fix race with folio split/free using temporary reference

[David Hildenbrand <david@redhat.com>]

On 27.05.25 05:20, Dev Jain wrote:
> 
> On 26/05/25 11:58 pm, Shivank Garg wrote:
>> hpage_collapse_scan_file() calls is_refcount_suitable(), which in turn
>> calls folio_mapcount(). folio_mapcount() checks folio_test_large() before
>> proceeding to folio_large_mapcount(), but there is a race window where the
>> folio may get split/freed between these checks, triggering:
>>
>>     VM_WARN_ON_FOLIO(!folio_test_large(folio), folio)
>>
>> Take a temporary reference to the folio in hpage_collapse_scan_file().
>> This stabilizes the folio during refcount check and prevents incorrect
>> large folio detection due to concurrent split/free. Use helper
>> folio_expected_ref_count() + 1 to compare with folio_ref_count()
>> instead of using is_refcount_suitable().
>>
>> Fixes: 05c5323b2a34 ("mm: track mapcount of large folios in single value")
>> Reported-by: syzbot+2b99589e33edbe9475ca@syzkaller.appspotmail.com
>> Closes: https://lore.kernel.org/all/6828470d.a70a0220.38f255.000c.GAE@google.com
>> Suggested-by: David Hildenbrand <david@redhat.com>
>> Acked-by: David Hildenbrand <david@redhat.com>
>> Signed-off-by: Shivank Garg <shivankg@amd.com>
>> ---
> 
> The patch looks fine.
> 
> I was just wondering about the implications of this on migration. Earlier
> we had a refcount race between migration and shmem page fault via filemap_get_entry()
> taking a reference and not releasing it till we take the folio lock, which was held
> by the migration path. I would like to *think* that real workloads, when migrating
> pages, will *not* be faulting on those pages simultaneously, just guessing. But now
> we have a kernel thread (khugepaged) racing against migration. I may just be over-speculating.

I'm not quite sure I understand the concern you have. Any temporary 
reference can temporarily block migration, however, the retry logic 
should be able to handle that just fine -- and this code is not really 
special (see filemap_get_entry()).

-- 
Cheers,

David / dhildenb