Re: [PATCH] mm/huge_memory: add folio_reset_partially_mapped()

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Mon, Sep 07, 2026 at 07:33:40PM +0100, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
>
> __folio_unqueue_deferred_split() and __folio_freeze_and_split_unmapped()
> both clear PG_partially_mapped and take the folio out of
> MTHP_STAT_NR_ANON_PARTIALLY_MAPPED with the same five lines.
>
> Move the block into folio_reset_partially_mapped() and call it from both
> places.
>
> The helper asserts what both callers rely on: the folio is frozen, so
> deferred_split_folio() cannot set the flag again under it, and the folio
> is already off the deferred split queue. The list check sits behind the
> flag test because order-1 folios have no _deferred_list.
>
> folio_order() is safe to use at this point in the split process: it
> still shows the pre-split order.
>
> Suggested-by: David Hildenbrand (Arm) <david@kernel.org>
> Assisted-by: Claude-Code:claude-fable-5-1

Note that policy has changed on Assisted-by tags so this should be:

Assisted-by: LLM

https://docs.kernel.org/process/coding-assistants.html

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

One nit above and below, with those addressed LGTM, so:

Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> ---
>  mm/huge_memory.c | 31 ++++++++++++++++++++-----------
>  1 file changed, 20 insertions(+), 11 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index dd66c6ad5af1..23ef22c2b5db 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3972,6 +3972,24 @@ static unsigned int folio_cache_ref_count(const struct folio *folio)
>  	return folio_nr_pages(folio);
>  }
>
> +static void folio_reset_partially_mapped(struct folio *folio)
> +{
> +	VM_WARN_ON_FOLIO(folio_ref_count(folio), folio);

NIT: I wonder if we could have a folio_is_frozen() that'd make this
self-documenting? As trivial as it'd be :)

If not, then a comment like:

	/* Folio must be frozen. */

Would be helpful.

> +
> +	if (!folio_test_partially_mapped(folio))
> +		return;
> +
> +	/*
> +	 * Order-1 folios have no _deferred_list. The flag is only ever set
> +	 * on folios that do, so the list can be checked after the flag.
> +	 */
> +	VM_WARN_ON_FOLIO(!list_empty(&folio->_deferred_list), folio);
> +
> +	folio_clear_partially_mapped(folio);
> +	mod_mthp_stat(folio_order(folio),
> +		      MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, -1);
> +}
> +
>  static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int new_order,
>  					     struct page *split_at, struct xa_state *xas,
>  					     struct address_space *mapping, bool do_lru,
> @@ -3980,7 +3998,6 @@ static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int n
>  {
>  	struct folio *end_folio = folio_next(folio);
>  	struct folio *new_folio, *next;
> -	int old_order = folio_order(folio);
>  	int ret = 0;
>
>  	VM_WARN_ON_ONCE(!mapping && end);
> @@ -3998,11 +4015,7 @@ static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int n
>  		 * leaves PG_partially_mapped set.
>  		 * Clear it here: the flag does not survive the split.
>  		 */
> -		if (folio_test_partially_mapped(folio)) {
> -			folio_clear_partially_mapped(folio);
> -			mod_mthp_stat(old_order,
> -				      MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, -1);
> -		}
> +		folio_reset_partially_mapped(folio);
>
>  		if (mapping) {
>  			int nr = folio_nr_pages(folio);
> @@ -4516,11 +4529,7 @@ bool __folio_unqueue_deferred_split(struct folio *folio)
>  	memcg = folio_memcg(folio);
>  	lru = list_lru_lock_irqsave(&deferred_split_lru, nid, &memcg, &flags);
>  	if (__list_lru_del(&deferred_split_lru, lru, &folio->_deferred_list, nid)) {
> -		if (folio_test_partially_mapped(folio)) {
> -			folio_clear_partially_mapped(folio);
> -			mod_mthp_stat(folio_order(folio),
> -				      MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, -1);
> -		}
> +		folio_reset_partially_mapped(folio);

Nice bit of red :)

>  		unqueued = true;
>  	}
>  	list_lru_unlock_irqrestore(lru, &flags);
>
> base-commit: e3fc12b08aadde9cec7b3799ac0e0c9a1aa245c4
> --
> 2.54.0
>

--
Cheers, Lorenzo