linux-kernel - Re: [PATCH 1/7] mm: fix folio_expected_ref_count() when PG_private

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Message-ID: <685fa6c1-e343-ae92-b673-48524918548d@google.com>
Date: Mon, 8 Sep 2025 12:47:59 -0700 (PDT)
From: Hugh Dickins <hughd@...gle.com>
To: Matthew Wilcox <willy@...radead.org>
cc: Hugh Dickins <hughd@...gle.com>, David Hildenbrand <david@...hat.com>, 
    Andrew Morton <akpm@...ux-foundation.org>, Will Deacon <will@...nel.org>, 
    Shivank Garg <shivankg@....com>, 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>, 
    David Howells <dhowells@...hat.com>, ceph-devel@...r.kernel.org, 
    linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 1/7] mm: fix folio_expected_ref_count() when
 PG_private_2

On Mon, 8 Sep 2025, Matthew Wilcox wrote:
> On Mon, Sep 08, 2025 at 03:27:47AM -0700, Hugh Dickins wrote:
> > On Mon, 1 Sep 2025, David Hildenbrand wrote:
> > > On 01.09.25 09:52, David Hildenbrand wrote:
> > > > On 01.09.25 03:17, Hugh Dickins wrote:
> > > >> On Mon, 1 Sep 2025, Matthew Wilcox wrote:
> > > >>> On Sun, Aug 31, 2025 at 02:01:16AM -0700, Hugh Dickins wrote:
> > > >>>> 6.16's folio_expected_ref_count() is forgetting the PG_private_2 flag,
> > > >>>> which (like PG_private, but not in addition to PG_private) counts for
> > > >>>> 1 more reference: it needs to be using folio_has_private() in place of
> > > >>>> folio_test_private().
> > > >>>
> > > >>> No, it doesn't.  I know it used to, but no filesystem was actually doing
> > > >>> that.  So I changed mm to match how filesystems actually worked.
> > 
> > I think Matthew may be remembering how he wanted it to behave (? but he
> > wanted it to go away completely) rather than how it ended up behaving:
> > we've both found that PG_private_2 always goes with refcount increment.
> 
> Let me explain that better.  No filesystem followed the documented rule
> that the refcount must be incremented by one if either PG_private or
> PG_private_2 was set.  And no surprise; that's a very complicated rule
> for filesystems to follow.  Many of them weren't even following the rule
> to increment the refcount by one when PG_private was set.

Thanks, yes, I hadn't realized that you were referring to the +1 (versus
+2) part of it: yes, a quite unnecessarily difficult rule to follow.

...

> So the current behaviour where we set private_2 and bump the refcount,
> but don't take the private_2 status into account is the safe one,
> because the elevated refcount means we'll skip the PG_fscache folio.
> Maybe it'd be better to wait for it to clear.  But since Dave Howells
> is busy killing it off, I'm just inclined to wait for that to happen.

Yes, that's where my internalized-Matthew brought me in the end;
though killing off PG_private_2 seems to have been just around
the corner for a very long time.

It's a pity that there isn't much incentive on ceph folks to
get it fixed: the one who suffers is the compactor or pinner.

Hugh