Re: [PATCH v2] mm/hugetlb: fix a deadlock with pagecache_folio and hugetlb_fault_mutex_table

[Gavin Guo <gavinguo@igalia.com>]

On 5/21/25 23:58, Hugh Dickins wrote:
> On Wed, 21 May 2025, Oscar Salvador wrote:
>> On Wed, May 21, 2025 at 08:10:46AM -0700, Hugh Dickins wrote:
>>> Unless you have a very strong argument why this folio is invisible to
>>> the rest of the world, including speculative accessors like compaction
>>> (and the name "pagecache_folio" suggests very much the reverse): the
>>> pattern of unlocking a lock when you see it locked is like (or worse
>>> than) having no locking at all - it is potentially unlocking someone
>>> else's lock.
>>
>> hugetlb_fault() locks 'pagecache_folio' and unlocks it after returning
>> from hugetlb_wp().
>> This patch introduces the possibility that hugetlb_wp() can also unlock it for
>> the reasons explained.
>> So, when hugetlb_wp() returns back to hugetlb_fault(), we
>>
>> 1) either still hold the lock (because hugetlb_fault() took it)
>> 2) or we do not anymore because hugetlb_wp() unlocked it for us.
>>
>> So it is not that we are unlocking anything blindly, because if the lock
>> is still 'taken' (folio_test_locked() returned true) it is because we,
>> hugetlb_fault() took it and we are still holding it.
> 
> If we unlocked it, anyone else could have taken it immediately after.
> 
> Hugh
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Sigh, I should have thought of that as well. Next time, I'll be more 
careful.