Re: [RFC PATCH 2/2] mm/huge_memory: detect partially mapped shared folios without per-page mapcounts

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Medium] mm/huge_memory: deferred_split_scan() runs PMD
  thp_underused() on mTHPs
--

commit 2f9d38ddfb170d3656f9925bef1e82e9b94ecc37
Author: Jose A. Perez de Azpillaga <azpijr@gmail.com>

mm/huge_memory: detect partially mapped shared folios without per-page mapcounts

This commit queues shared anonymous folios onto the deferred split list
without setting PG_partially_mapped when CONFIG_NO_PAGE_MAPCOUNT cannot tell
at unmap time whether all subpages remain mapped, and walks the rmap in
deferred_split_scan() to check them.

> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 8c4a69c797edd..f80fa028dd340 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c

[ ... ]

> @@ -4744,6 +4811,15 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
>  		list_lru_unlock_irq(lru);
>  		rcu_read_unlock();
>  
> +		if (!folio_test_partially_mapped(folio) &&
> +		    folio_needs_mapped_check(folio)) {
> +			if (!folio_trylock(folio))
> +				goto requeue;
> +			/* Sets PG_partially_mapped and puts it back on the queue. */
> +			if (folio_mapped_shared_partially(folio))
> +				deferred_split_folio(folio, true);
> +			folio_unlock(folio);
> +		}
>  		if (!folio_test_partially_mapped(folio)) {
>  			/*
>  			 * See try_to_map_unused_to_zeropage(): we cannot
> @@ -4752,7 +4828,7 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
>  			 */
>  			if (folio_test_mlocked(folio))
>  				goto next;
> -			underused = thp_underused(folio);
> +			underused = split_underused_thp && thp_underused(folio);
>  			if (!underused)
>  				goto next;
>  		}

[Severity: Medium]
Can deferred_split_scan() now call thp_underused() on a sub-PMD mTHP?

Previously, only map_anon_folio_pmd_nopf() queued folios with
partially_mapped == false, so any folio reaching the !folio_test_partially_mapped(folio)
branch was PMD-sized.  __folio_remove_rmap() now queues shared anonymous
folios of any order > 1 with partially_mapped == false when a partial unmap in
one process leaves the folio fully mapped in another.  When
folio_mapped_shared_partially() returns false for such a sub-PMD mTHP,
execution falls through to thp_underused(folio):

mm/huge_memory.c:thp_underused() {
    ...
	for (i = 0; i < folio_nr_pages(folio); i++) {
		if (pages_identical(folio_page(folio, i), ZERO_PAGE(0))) {
			if (++num_zero_pages > khugepaged_max_ptes_none)
				return true;
		} else {
			if (++num_filled_pages >= HPAGE_PMD_NR - khugepaged_max_ptes_none)
				return false;
		}
	}
    ...
}

Because thp_underused() compares against unscaled HPAGE_PMD_NR and
khugepaged_max_ptes_none thresholds, any khugepaged_max_ptes_none below
HPAGE_PMD_NR - 1 causes reclaim to scan subpages with pages_identical() even
when neither threshold can be reached, or to split fully mapped sub-PMD mTHPs
and increment THP_UNDERUSED_SPLIT_PAGE.  Checking folio_test_pmd_mappable(folio)
before calling thp_underused(folio) would avoid this.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261111113535.330078-1-azpijr@gmail.com?part=2