Re: [PATCH] Revert mm/gup: small refactoring: simplify try_grab_page()

[Will McVicker <willmcvicker@google.com>]

On Tue, Feb 1, 2022 at 10:00 AM Will McVicker <willmcvicker@google.com> wrote:
>
> On Tue, Feb 1, 2022 at 1:29 AM John Hubbard <jhubbard@nvidia.com> wrote:
> >
> > This reverts commit 54d516b1d62ff8f17cee2da06e5e4706a0d00b8a
> >
> > That commit did a refactoring that effectively combined fast and slow
> > gup paths (again). And that was again incorrect, for two reasons:
> >
> > a) Fast gup and slow gup get reference counts on pages in different ways
> > and with different goals: see Linus' writeup in commit cd1adf1b63a1
> > ("Revert "mm/gup: remove try_get_page(), call try_get_compound_head()
> > directly""), and
> >
> > b) try_grab_compound_head() also has a specific check for "FOLL_LONGTERM
> > && !is_pinned(page)", that assumes that the caller can fall back to slow
> > gup. This resulted in new failures, as recently report by Will McVicker
> > [1].
> >
> > But (a) has problems too, even though they may not have been reported
> > yet. So just revert this.
> >
> > [1] https://lore.kernel.org/r/20220131203504.3458775-1-willmcvicker@google.com
> >
> > Fixes: 54d516b1d62f ("mm/gup: small refactoring: simplify try_grab_page()")
> > Cc: Christoph Hellwig <hch@lst.de>
> > Cc: Will McVicker <willmcvicker@google.com>
> > Cc: Minchan Kim <minchan@google.com>
> > Cc: Matthew Wilcox <willy@infradead.org>
> > Cc: Christian Borntraeger <borntraeger@de.ibm.com>
> > Cc: Heiko Carstens <hca@linux.ibm.com>
> > Cc: Vasily Gorbik <gor@linux.ibm.com>
> > Cc: Linus Torvalds <torvalds@linux-foundation.org>
> > Signed-off-by: John Hubbard <jhubbard@nvidia.com>
> > ---
> >  mm/gup.c | 35 ++++++++++++++++++++++++++++++-----
> >  1 file changed, 30 insertions(+), 5 deletions(-)
> >
> > diff --git a/mm/gup.c b/mm/gup.c
> > index f0af462ac1e2..a9d4d724aef7 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -124,8 +124,8 @@ static inline struct page *try_get_compound_head(struct page *page, int refs)
> >   * considered failure, and furthermore, a likely bug in the caller, so a warning
> >   * is also emitted.
> >   */
> > -struct page *try_grab_compound_head(struct page *page,
> > -                                   int refs, unsigned int flags)
> > +__maybe_unused struct page *try_grab_compound_head(struct page *page,
> > +                                                  int refs, unsigned int flags)
> >  {
> >         if (flags & FOLL_GET)
> >                 return try_get_compound_head(page, refs);
> > @@ -208,10 +208,35 @@ static void put_compound_head(struct page *page, int refs, unsigned int flags)
> >   */
> >  bool __must_check try_grab_page(struct page *page, unsigned int flags)
> >  {
> > -       if (!(flags & (FOLL_GET | FOLL_PIN)))
> > -               return true;
> > +       WARN_ON_ONCE((flags & (FOLL_GET | FOLL_PIN)) == (FOLL_GET | FOLL_PIN));
> >
> > -       return try_grab_compound_head(page, 1, flags);
> > +       if (flags & FOLL_GET)
> > +               return try_get_page(page);
> > +       else if (flags & FOLL_PIN) {
> > +               int refs = 1;
> > +
> > +               page = compound_head(page);
> > +
> > +               if (WARN_ON_ONCE(page_ref_count(page) <= 0))
> > +                       return false;
> > +
> > +               if (hpage_pincount_available(page))
> > +                       hpage_pincount_add(page, 1);
> > +               else
> > +                       refs = GUP_PIN_COUNTING_BIAS;
> > +
> > +               /*
> > +                * Similar to try_grab_compound_head(): even if using the
> > +                * hpage_pincount_add/_sub() routines, be sure to
> > +                * *also* increment the normal page refcount field at least
> > +                * once, so that the page really is pinned.
> > +                */
> > +               page_ref_add(page, refs);
> > +
> > +               mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_ACQUIRED, 1);
> > +       }
> > +
> > +       return true;
> >  }
> >
> >  /**
> >
> > base-commit: 26291c54e111ff6ba87a164d85d4a4e134b7315c
> > --
> > 2.35.1
> >
>
> Thanks John! I verified this works on the Pixel 6 with the 5.15 kernel
> for my camera use-case. Free free to include:
>
> Tested-by: Will McVicker <willmcvicker@google.com>
>
> Thanks,
> Will

And just so we don't miss this, I'd also like to request this be
pulled into the 5.15 stable branch please.

Cc: stable@vger.kernel.org # 5.15

Thanks,
Will