Re: [PATCH v2 2/6] mm/gup: local lru_add_drain() to avoid lru_add_drain_all()

[David Hildenbrand]

On 09.09.25 00:16, Hugh Dickins wrote:
> In many cases, if collect_longterm_unpinnable_folios() does need to
> drain the LRU cache to release a reference, the cache in question is
> on this same CPU, and much more efficiently drained by a preliminary
> local lru_add_drain(), than the later cross-CPU lru_add_drain_all().
>
> Marked for stable, to counter the increase in lru_add_drain_all()s
> from "mm/gup: check ref_count instead of lru before migration".
> Note for clean backports: can take 6.16 commit a03db236aebf ("gup:
> optimize longterm pin_user_pages() for large folio") first.
>
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> ---
>   mm/gup.c | 15 +++++++++++----
>   1 file changed, 11 insertions(+), 4 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 82aec6443c0a..b47066a54f52 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2287,8 +2287,8 @@ static unsigned long collect_longterm_unpinnable_folios(
>   		struct pages_or_folios *pofs)
>   {
>   	unsigned long collected = 0;
> -	bool drain_allow = true;
>   	struct folio *folio;
> +	int drained = 0;
>   	long i = 0;
>   
>   	for (folio = pofs_get_folio(pofs, i); folio;
> @@ -2307,10 +2307,17 @@ static unsigned long collect_longterm_unpinnable_folios(
>   			continue;
>   		}
>   
> -		if (drain_allow && folio_ref_count(folio) !=
> -				   folio_expected_ref_count(folio) + 1) {
> +		if (drained == 0 &&
> +				folio_ref_count(folio) !=
> +				folio_expected_ref_count(folio) + 1) {

I would just have indented this as follows:

		if (drained == 0 &&
		    folio_ref_count(folio) != folio_expected_ref_count(folio) + 1) {

Same below.

In any case logic LGTM

Acked-by: David Hildenbrand <david@xxxxxxxxxx>

--
Cheers

David / dhildenb