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

[Hugh Dickins <hughd@google.com>]

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