Re: [PATCH 02/15] mm/hugetlb: compare against persistent count when freeing lowmem folios

[Muchun Song]

> On Oct 29, 2026, at 04:13, David Hildenbrand (Arm) <david@xxxxxxxxxx> wrote:
> 
> try_to_free_low() gets a target number of persistent folios, but
> compares it with "nr_huge_pages", which includes surplus folios. As a
> result, it can free lowmem folios below that target.
> 
> For example, with two persistent folios and one surplus folio, a target
> of two can still end up freeing one persistent folio.
> 
> Compare against persistent_huge_pages() instead. Convert the macro to a
> helper function and place it before try_to_free_low().
> 
> Fixes: 7893d1d505d5 ("hugetlb: Try to grow hugetlb pool for MAP_PRIVATE mappings")
> Cc: stable@xxxxxxxxxx
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

Acked-by: Muchun Song <muchun.song@xxxxxxxxx>

One nit bellow.

> ---
> mm/hugetlb.c | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index da5d04edf7c1..3a94ea1c9457 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1335,6 +1335,11 @@ static unsigned long available_huge_pages(struct hstate *h)
> 	return h->free_huge_pages - h->resv_huge_pages;
> }
> 
> +static unsigned long persistent_huge_pages(struct hstate *h)

Better to make *h const?

Thanks.

> +{
> +	return h->nr_huge_pages - h->surplus_huge_pages;
> +}
> +
> static struct folio *dequeue_hugetlb_folio(struct hstate *h, gfp_t gfp_mask,
> 					   struct mempolicy_interpreted *mpoli)
> {
> @@ -3719,7 +3724,7 @@ static void try_to_free_low(struct hstate *h, unsigned long count,
> 		struct folio *folio, *next;
> 		struct list_head *freel = &h->hugepage_freelists[i];
> 		list_for_each_entry_safe(folio, next, freel, lru) {
> -			if (count >= h->nr_huge_pages)
> +			if (count >= persistent_huge_pages(h))
> 				goto out;
> 			if (folio_test_highmem(folio))
> 				continue;
> @@ -3773,7 +3778,6 @@ static int adjust_pool_surplus(struct hstate *h, nodemask_t *nodes_allowed,
> 	return 1;
> }
> 
> -#define persistent_huge_pages(h) (h->nr_huge_pages - h->surplus_huge_pages)
> static int set_max_huge_pages(struct hstate *h, unsigned long count, int nid,
> 			      nodemask_t *nodes_allowed)
> {
> 
> -- 
> 2.43.0
>