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

[Hugh Dickins <hughd@google.com>]

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'm not sure if there's still documentation lying around that gets
> this wrong or if you're remembering how things used to be documented,
> but it's never how any filesystem has ever worked.
> 
> We're achingly close to getting rid of PG_private_2.  I think it's just
> ceph and nfs that still use it.

I knew you were trying to get rid of it (hurrah! thank you), so when I
tried porting my lru_add_drainage to 6.12 I was careful to check whether
folio_expected_ref_count() would need to add it to the accounting there:
apparently yes; but then I was surprised to find that it's still present
in 6.17-rc, I'd assumed it gone long ago.

I didn't try to read the filesystems (which could easily have been
inconsistent about it) to understand: what convinced me amidst all
the confusion was this comment and code in mm/filemap.c:

/**
 * folio_end_private_2 - Clear PG_private_2 and wake any waiters.
 * @folio: The folio.
 *
 * Clear the PG_private_2 bit on a folio and wake up any sleepers waiting for
 * it.  The folio reference held for PG_private_2 being set is released.
 *
 * This is, for example, used when a netfs folio is being written to a local
 * disk cache, thereby allowing writes to the cache for the same folio to be
 * serialised.
 */
void folio_end_private_2(struct folio *folio)
{
	VM_BUG_ON_FOLIO(!folio_test_private_2(folio), folio);
	clear_bit_unlock(PG_private_2, folio_flags(folio, 0));
	folio_wake_bit(folio, PG_private_2);
	folio_put(folio);
}
EXPORT_SYMBOL(folio_end_private_2);

That seems to be clear that PG_private_2 is matched by a folio reference,
but perhaps you can explain it away - worth changing the comment if so.

I was also anxious to work out whether PG_private with PG_private_2
would mean +1 or +2: I don't think I found any decisive statement,
but traditional use of page_has_private() implied +1; and I expect
there's no filesystem which actually could have both on the same folio.

Thanks,
Hugh