Re: [PATCH v2 1/1] mm: skip mlocked THPs that are underused early in deferred_split_scan()

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Mon, Sep 08, 2025 at 05:07:41PM +0800, Lance Yang wrote:
> From: Lance Yang <lance.yang@linux.dev>
>
> When we stumble over a fully-mapped mlocked THP in the deferred shrinker,
> it does not make sense to try to detect whether it is underused, because
> try_to_map_unused_to_zeropage(), called while splitting the folio, will not
> actually replace any zeroed pages by the shared zeropage.
>

I see it's along this lovely code path: :)

deferred_split_scan()
(thp_underused() returns true)
-> split_folio()
-> split_folio_to_list()
-> split_huge_page_to_list_to_order()
-> __folio_split()
-> remap_page()
-> remove_migration_ptes()
-> remove_migration_pte()
-> try_to_map_unused_to_zeropage()
(oops mlock'd)

> Splitting the folio in that case does not make any sense, so let's not even
> scan to check if the folio is underused.
>
> Suggested-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Lance Yang <lance.yang@linux.dev>

Anyway this does make a lot of sense! So:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> ---
> v1 -> v2:
>  - Update subject and changelog (per David, thanks!)
>  - https://lore.kernel.org/linux-mm/20250908040713.42960-1-lance.yang@linux.dev
>
>  mm/huge_memory.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 77f0c3417973..6d99893c18e8 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -4183,6 +4183,13 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
>  		bool underused = false;
>
>  		if (!folio_test_partially_mapped(folio)) {
> +			/*
> +			 * See try_to_map_unused_to_zeropage(): we cannot
> +			 * optimize zero-filled pages after splitting an
> +			 * mlocked folio.
> +			 */
> +			if (folio_test_mlocked(folio))
> +				goto next;
>  			underused = thp_underused(folio);
>  			if (!underused)
>  				goto next;
> --
> 2.49.0
>