Re: [PATCH v3 2/3] mm/huge_memory: don't queue folios when underused splitting is inactive

[Kiryl Shutsemau <kas@kernel.org>]

On Fri, Oct 23, 2026 at 04:00:05PM -0700, Joanne Koong wrote:
> Every anonymous PMD-sized folio is put on the deferred split queue when
> it is first mapped, so that the shrinker can find it under memory
> pressure and split it if it turns out to be mostly zero-filled.
> 
> With the default khugepaged/max_ptes_none this is wasted work. The
> default is HPAGE_PMD_NR - 1, which tells thp_underused() that any number
> of zero-filled pages is tolerable, which means the shrinker will never
> split any of these folios for being underused. They take the list_lru
> lock at fault time, inflate the object count the shrinker reports, and
> are then walked and dropped when they're first scanned.
> 
> Skip the queueing when khugepaged/max_ptes_none is at its maximum, and
> factor that check out into thp_underused_never() so that
> deferred_split_folio() and thp_underused() share it. Since the default
> is the maximum, out of the box no folio is queued for underused
> splitting at all, PMD-sized ones included. Partially mapped folios are
> still queued as before.
> 
> This also matters for a subsequent patch that adds anonymous mTHP folios
> to the deferred split list, as it keeps systems running the default from
> paying for that on every anonymous fault.
> 
> Please note that the queue has always been best-effort. A folio that was
> queued before khugepaged/max_ptes_none is lowered gets dropped from the
> queue by the first scan that finds it's not underused, so changing the
> sysctl has never retroactively applied to folios that were already
> scanned. Requeueing eligible folios when the sysctl changes will be
> addressed in a separate patch.
> 
> Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
> Suggested-by: David Hildenbrand <david@kernel.org>
> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>

Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov