Re: [RESEND PATCH] mm/madvise: use folio_trylock() in the cold/pageout PMD split

[Gregory Price <gourry@gourry.net>]

On Wed, Sep 16, 2026 at 08:25:01AM +0200, David Hildenbrand (Arm) wrote:
> On 9/12/26 13:05, Gregory Price wrote:
> > MADV_COLD or MADV_PAGEOUT over part of a PMD splits the THP in
> > madvise_cold_or_pageout_pte_range().  Two threads doing that to
> > the same THP create spurious failures.
> > 
> >   CPU0                          CPU1
> >   ----                          ----
> >   folio_get()
> >   spin_unlock(ptl)
> >   folio_lock()
> >                                 folio_get()
> >                                 spin_unlock(ptl)
> >                                 folio_lock()  <- blocks, keeps its ref
> >   split_folio()
> >     folio_expected_ref_count(folio) != folio_ref_count(folio) - 1
> >     -EAGAIN
> > 
> > CPU1 cannot drop its reference until it gets the lock CPU0 holds, so CPU0's
> > split always fails.  folio_trylock() makes CPU1 leave without ever taking a
> > reference.  The PTE branch of this same function already does this, as do
> > madvise_free_pte_range() and madvise_free_huge_pmd().
> > 
> > Reproducer: 400 rounds of eight threads calling MADV_COLD on half of each
> > of eight THPs, re-formed with MADV_COLLAPSE between rounds.  From
> > /proc/vmstat:
> > 
> >                      thp_split_page   thp_split_page_failed
> >     before                     3186                     860
> >     after                      3200                       0
> > 
> > The short before count is rounds where every thread failed and the
> > advice was dropped for that THP entirely.

bleh i thought i rewrote this sentence.

reading madvise is driving me crazy.

> 
> While the split now succeeds, one of both calls will just effectively skip
> processing the page table. SO while CPU0 will succeed with the split, CPU1 would
> just skip the page table.
> 
> And what happened before?
> 
> Split on CPU0 failed and it would skip the page table. Split on CPU1, however,
> would likely have succeeded?
> 

Yes., all correct.

> So this is all far from perfect.
> 
> ... but this matches what we do further down in the PTE scenario.
> 
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>
>

Thank you!
~gregory