Re: [PATCH 2/7] mm/gup: check ref_count instead of lru before migration

[Hugh Dickins]

On Mon, 1 Sep 2025, David Hildenbrand wrote:
> On 31.08.25 11:05, Hugh Dickins wrote:
> > diff --git a/mm/gup.c b/mm/gup.c
> > index adffe663594d..82aec6443c0a 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -2307,7 +2307,8 @@ static unsigned long
> > collect_longterm_unpinnable_folios(
> >     	continue;
> >     }
> >   -		if (!folio_test_lru(folio) && drain_allow) {
> > +		if (drain_allow && folio_ref_count(folio) !=
> > +				   folio_expected_ref_count(folio) + 1) {
> >      lru_add_drain_all();
> >      drain_allow = false;
> >     }
> 
> In general, to the fix idea
> 
> 	Acked-by: David Hildenbrand <david@xxxxxxxxxx>

Thanks, but I'd better not assume that in v2, even though code the same.
Will depend on how you feel about added paragraph in v2 commit message.

> 
> But as raised in reply to patch #1, we have to be a bit careful about
> including private_2 in folio_expected_ref_count() at this point.
> 
> If we cannot include it in folio_expected_ref_count(), it's all going to be a
> mess until PG_private_2 is removed for good.
> 
> So that part still needs to be figured out.

Here's that added paragraph:

Note on PG_private_2: ceph and nfs are still using the deprecated
PG_private_2 flag, with the aid of netfs and filemap support functions.
Although it is consistently matched by an increment of folio ref_count,
folio_expected_ref_count() intentionally does not recognize it, and ceph
folio migration currently depends on that for PG_private_2 folios to be
rejected.  New references to the deprecated flag are discouraged, so do
not add it into the collect_longterm_unpinnable_folios() calculation:
but longterm pinning of transiently PG_private_2 ceph and nfs folios
(an uncommon case) may invoke a redundant lru_add_drain_all().  And
this makes easy the backport to earlier releases: up to and including
6.12, btrfs also used PG_private_2, but without a ref_count increment.

Hugh