Revert mm/gup: small refactoring: simplify try_grab_page()

Commit Message

John Hubbard Feb. 1, 2022, 9:29 a.m. UTC

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/[e-mail]

Fixes: 54d516b1d62f ("mm/gup: small refactoring: simplify try_grab_page()")
Cc: Christoph Hellwig <[e-mail]>
Cc: Will McVicker <[e-mail]>
Cc: Minchan Kim <[e-mail]>
Cc: Matthew Wilcox <[e-mail]>
Cc: Christian Borntraeger <[e-mail]>
Cc: Heiko Carstens <[e-mail]>
Cc: Vasily Gorbik <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Signed-off-by: John Hubbard <[e-mail]>
---
 mm/gup.c | 35 ++++++++++++++++++++++++++++++-----
 1 file changed, 30 insertions(+), 5 deletions(-)


base-commit: 26291c54e111ff6ba87a164d85d4a4e134b7315c

Comments

William McVicker Feb. 1, 2022, 6 p.m. UTC | #1

On Tue, Feb 1, 2022 at 1:29 AM John Hubbard <[e-mail]> 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/[e-mail]
>
> Fixes: 54d516b1d62f ("mm/gup: small refactoring: simplify try_grab_page()")
> Cc: Christoph Hellwig <[e-mail]>
> Cc: Will McVicker <[e-mail]>
> Cc: Minchan Kim <[e-mail]>
> Cc: Matthew Wilcox <[e-mail]>
> Cc: Christian Borntraeger <[e-mail]>
> Cc: Heiko Carstens <[e-mail]>
> Cc: Vasily Gorbik <[e-mail]>
> Cc: Linus Torvalds <[e-mail]>
> Signed-off-by: John Hubbard <[e-mail]>
> ---
>  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 <[e-mail]>

Thanks,
Will

William McVicker Feb. 1, 2022, 6:32 p.m. UTC | #2

On Tue, Feb 1, 2022 at 10:00 AM Will McVicker <[e-mail]> wrote:
>
> On Tue, Feb 1, 2022 at 1:29 AM John Hubbard <[e-mail]> 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/[e-mail]
> >
> > Fixes: 54d516b1d62f ("mm/gup: small refactoring: simplify try_grab_page()")
> > Cc: Christoph Hellwig <[e-mail]>
> > Cc: Will McVicker <[e-mail]>
> > Cc: Minchan Kim <[e-mail]>
> > Cc: Matthew Wilcox <[e-mail]>
> > Cc: Christian Borntraeger <[e-mail]>
> > Cc: Heiko Carstens <[e-mail]>
> > Cc: Vasily Gorbik <[e-mail]>
> > Cc: Linus Torvalds <[e-mail]>
> > Signed-off-by: John Hubbard <[e-mail]>
> > ---
> >  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 <[e-mail]>
>
> 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: [e-mail] # 5.15

Thanks,
Will

John Hubbard Feb. 1, 2022, 8:38 p.m. UTC | #3

On 2/1/22 10:32, Will McVicker wrote:
...
>> 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 <[e-mail]>
>>
>> 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: [e-mail] # 5.15
> 
> Thanks,
> Will
> 

Yes. Let's see if any feedback shows up about the approach, otherwise
I'll post a v2 in a day or so, that adds your tested-by, as well as the
Cc: stable.


thanks,

Patch

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;
 }
 
 /**