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

[Kiryl Shutsemau]

On Tue, Sep 09, 2025 at 09:56:30AM +0200, David Hildenbrand wrote:
> 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) {

Do we want folio_check_expected_ref_count(folio, offset)?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov