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 10:06, Dev Jain wrote:
> 
> On 27/05/25 1:18 pm, David Hildenbrand wrote:
>> 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()).
> 
> 
> You are correct that any temp ref can block migration, however, that
> reference has to come after the folios have been isolated in the
> migration path.
> 
> So the probability of someone taking a reference on the folio is quite
> low since it has been isolated. The problem with filemap_get_entry() is
> that it finds
> 
> the folio in the pagecache, so isolation is useless, then takes a
> reference and then shmem_get_folio_gfp() does a folio_lock() instead of
> folio_try_lock().
> 
> This was the race which I talked about an year back at [1]. My concern
> is that we are adding another candidate to that race; just wondering if
> there is
> 
> a better solution to fix the race mentioned in Shivank's patchset.

Note that in this code here, we're not locking the folio just yet, we're 
only grabbing a reference for a very short time.

We will isolate+lock the folios in collapse_file(), where we also lock 
all slots in the pagecache.

The alternative would be to also lock all slots here, which is arguably 
worse.

-- 
Cheers,

David / dhildenb