Re: [PATCH 1/7] mm: fix folio_expected_ref_count() when PG_private_2

[Hugh Dickins <hughd@google.com>]

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