2022-02-02 10:41:40

by John Hubbard

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

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/[email protected]

Fixes: 54d516b1d62f ("mm/gup: small refactoring: simplify try_grab_page()")
Cc: Christoph Hellwig <[email protected]>
Cc: Will McVicker <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Christian Borntraeger <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: Vasily Gorbik <[email protected]>
Cc: Linus Torvalds <[email protected]>
Signed-off-by: John Hubbard <[email protected]>
---
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

2022-02-02 10:42:44

by William McVicker

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

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

Thanks,
Will

2022-02-02 11:12:49

by John Hubbard

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

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 <[email protected]>
>>
>> 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: [email protected] # 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,
--
John Hubbard
NVIDIA

2022-02-02 14:43:54

by William McVicker

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

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

Thanks,
Will