Re: [PATCH v4 4/4] mm: add anonymous mTHP folios to the deferred split list

[Kiryl Shutsemau <kas@kernel.org>]

On Wed, Nov 04, 2026 at 03:50:58PM -0800, Joanne Koong wrote:
> On Wed, Nov 4, 2026 at 1:21 PM <sashiko-bot@kernel.org> wrote:
> >
> > Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> > - [High] mm: try_to_migrate_one() triggers deferred_split_folio() on underused mTHP
> > --
> 
> Hi Sashiko,
> 
> I'll try to address these points below.
> 
> >
> > commit 82b7b7063b986d802fe52e75e17211e58f3f420c
> > Author: Joanne Koong <joannelkoong@gmail.com>
> >
> > mm: add anonymous mTHP folios to the deferred split list
> >
> > This commit queues newly mapped anonymous mTHP folios of at least 2M onto the
> > deferred split list in map_anon_folio_pte_nopf() so that underused mTHP folios
> > can be split by the deferred split shrinker under memory pressure.
> >
> > > diff --git a/mm/memory.c b/mm/memory.c
> > > index 0c9ed04528d88..44376321180b5 100644
> > > --- a/mm/memory.c
> > > +++ b/mm/memory.c
> > > @@ -5477,6 +5477,8 @@ void map_anon_folio_pte_nopf(struct folio *folio, pte_t *pte,
> > >       folio_add_lru_vma(folio, vma);
> > >       set_ptes(vma->vm_mm, addr, pte, entry, nr_pages);
> > >       update_mmu_cache_range(NULL, vma, addr, pte, nr_pages);
> > > +     if (folio_test_large(folio))
> > > +             deferred_split_folio(folio, false);
> > >  }
> >
> > [Severity: High]
> > When deferred_split_scan() splits an underused PTE-mapped mTHP folio queued
> > here with PG_partially_mapped left clear, doesn't unmap_folio() trigger a
> > false-positive deferred_split_folio(folio, true) transition during the split?
> 
> Yes, but I don't think this is specific to this series. Splitting any
> PTE-mapped anon large folio that isn't already marked partially mapped
> goes through the try_to_migrate_one() -> folio_remove_rmap_pte() path
> you describe below, where __folio_remove_rmap() requeues it as
> partially mapped mid-split. That already happens today (eg underused
> splits of PMD THPs whose PMD has been split). This series makes it
> more frequent since underused mTHP splits now take that path too, but
> this issue is pre-existing.

I think it is a bit of understatement.

For mTHP it is every underused split: the folio is always PTE-mapped and
try_to_migrate_one() drops one PTE at a time. Each underused split bumps
split_deferred for that size next to the new split_underused, and a split
that fails at folio_ref_freeze() leaves a fully mapped folio flagged
partially mapped on the queue, which the next scan splits without
calling thp_underused().

Can the fix go in before this series? I think it is hard requirement.
Let's not brush if off as pre-existing.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov