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

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Message-ID: <b008aef2-f69f-e2fb-d4d7-71a42d308529@google.com>
Date: Mon, 8 Sep 2025 03:40:47 -0700 (PDT)
From: Hugh Dickins <hughd@...gle.com>
To: David Hildenbrand <david@...hat.com>
cc: Hugh Dickins <hughd@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, 
    Will Deacon <will@...nel.org>, Shivank Garg <shivankg@....com>, 
    Matthew Wilcox <willy@...radead.org>, 
    Christoph Hellwig <hch@...radead.org>, Keir Fraser <keirf@...gle.com>, 
    Jason Gunthorpe <jgg@...pe.ca>, John Hubbard <jhubbard@...dia.com>, 
    Frederick Mayle <fmayle@...gle.com>, Peter Xu <peterx@...hat.com>, 
    "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>, 
    Johannes Weiner <hannes@...xchg.org>, Vlastimil Babka <vbabka@...e.cz>, 
    Alexander Krabler <Alexander.Krabler@...a.com>, 
    Ge Yang <yangge1116@....com>, Li Zhe <lizhe.67@...edance.com>, 
    Chris Li <chrisl@...nel.org>, Yu Zhao <yuzhao@...gle.com>, 
    Axel Rasmussen <axelrasmussen@...gle.com>, 
    Yuanchu Xie <yuanchu@...gle.com>, Wei Xu <weixugc@...gle.com>, 
    Konstantin Khlebnikov <koct9i@...il.com>, linux-kernel@...r.kernel.org, 
    linux-mm@...ck.org
Subject: Re: [PATCH 2/7] mm/gup: check ref_count instead of lru before
 migration

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@...hat.com>

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