| Message ID | 20210811070542.3403116-4-jhubbard@nvidia.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | A few gup refactorings and documentation updates | expand |
I agree that try_get_page() should probably be removed entirely; is there
a reason you didn't in v2 of the patch?
I'm also curious why you changed try_get_compound_head() into a routine
from an inline.
If you want to retain try_get_page() it should be an inline as well, especially
in its current implementation.
William Kucharski
> On Aug 11, 2021, at 1:05 AM, John Hubbard <jhubbard@nvidia.com> wrote:
>
> try_get_page() is very similar to try_get_compound_head(), and in fact
> try_get_page() has fallen a little behind in terms of maintenance:
> try_get_compound_head() handles speculative page references more
> thoroughly.
>
> There are only two try_get_page() callsites, so just call
> try_get_compound_head() directly from those, and remove try_get_page()
> entirely.
>
> Also, seeing as how this changes try_get_compound_head() into a
> non-static function, provide some kerneldoc documentation for it.
>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Matthew Wilcox <willy@infradead.org>
> Signed-off-by: John Hubbard <jhubbard@nvidia.com>
> ---
> arch/s390/mm/fault.c | 2 +-
> fs/pipe.c | 2 +-
> include/linux/mm.h | 10 +---------
> mm/gup.c | 21 +++++++++++++++++----
> 4 files changed, 20 insertions(+), 15 deletions(-)
>
> diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
> index 212632d57db9..fe1d2c1dbe3b 100644
> --- a/arch/s390/mm/fault.c
> +++ b/arch/s390/mm/fault.c
> @@ -822,7 +822,7 @@ void do_secure_storage_access(struct pt_regs *regs)
> break;
> case KERNEL_FAULT:
> page = phys_to_page(addr);
> - if (unlikely(!try_get_page(page)))
> + if (unlikely(try_get_compound_head(page, 1) == NULL))
> break;
> rc = arch_make_page_accessible(page);
> put_page(page);
> diff --git a/fs/pipe.c b/fs/pipe.c
> index 8e6ef62aeb1c..06ba9df37410 100644
> --- a/fs/pipe.c
> +++ b/fs/pipe.c
> @@ -191,7 +191,7 @@ EXPORT_SYMBOL(generic_pipe_buf_try_steal);
> */
> bool generic_pipe_buf_get(struct pipe_inode_info *pipe, struct pipe_buffer *buf)
> {
> - return try_get_page(buf->page);
> + return try_get_compound_head(buf->page, 1) != NULL;
> }
> EXPORT_SYMBOL(generic_pipe_buf_get);
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index ce8fc0fd6d6e..cd00d1222235 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1207,15 +1207,7 @@ bool __must_check try_grab_page(struct page *page, unsigned int flags);
> __maybe_unused struct page *try_grab_compound_head(struct page *page, int refs,
> unsigned int flags);
>
> -
> -static inline __must_check bool try_get_page(struct page *page)
> -{
> - page = compound_head(page);
> - if (WARN_ON_ONCE(page_ref_count(page) <= 0))
> - return false;
> - page_ref_inc(page);
> - return true;
> -}
> +struct page *try_get_compound_head(struct page *page, int refs);
>
> /**
> * folio_put - Decrement the reference count on a folio.
> diff --git a/mm/gup.c b/mm/gup.c
> index 64798d6b5043..c2d19d370c99 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -62,11 +62,24 @@ static void put_page_refs(struct page *page, int refs)
> put_page(page);
> }
>
> -/*
> - * Return the compound head page with ref appropriately incremented,
> - * or NULL if that failed.
> +/**
> + * try_get_compound_head() - return the compound head page with refcount
> + * appropriately incremented, or NULL if that failed.
> + *
> + * This handles potential refcount overflow correctly. It also works correctly
> + * for various lockless get_user_pages()-related callers, due to the use of
> + * page_cache_add_speculative().
> + *
> + * Even though the name includes "compound_head", this function is still
> + * appropriate for callers that have a non-compound @page to get.
> + *
> + * @page: pointer to page to be gotten
> + * @refs: the value to add to the page's refcount
> + *
> + * Return: head page (with refcount appropriately incremented) for success, or
> + * NULL upon failure.
> */
> -static inline struct page *try_get_compound_head(struct page *page, int refs)
> +struct page *try_get_compound_head(struct page *page, int refs)
> {
> struct page *head = compound_head(page);
>
> --
> 2.32.0
>
>
On 8/11/21 1:35 PM, William Kucharski wrote: > I agree that try_get_page() should probably be removed entirely; is there > a reason you didn't in v2 of the patch? Hi William, This patch *does* remove try_get_page() entirely! Look below. I'll reply inline, below, to show where that happens. > > I'm also curious why you changed try_get_compound_head() into a routine > from an inline. It was part of the change to make it available to callers outside of gup.c. try_get_compound_head() is slightly messy and doesn't like to live in mm.h, because it calls page_cache_add_speculative(), which lives in linux-pagemap.h, which, in turn, has its own set of different headers that it pulls in. So, leaving it in gup.c, and exposing it to the other callers as a non-static function, seemed appropriate here. > > If you want to retain try_get_page() it should be an inline as well, especially > in its current implementation. > > William Kucharski > >> On Aug 11, 2021, at 1:05 AM, John Hubbard <jhubbard@nvidia.com> wrote: >> >> try_get_page() is very similar to try_get_compound_head(), and in fact >> try_get_page() has fallen a little behind in terms of maintenance: >> try_get_compound_head() handles speculative page references more >> thoroughly. >> >> There are only two try_get_page() callsites, so just call >> try_get_compound_head() directly from those, and remove try_get_page() >> entirely. >> >> Also, seeing as how this changes try_get_compound_head() into a >> non-static function, provide some kerneldoc documentation for it. >> >> Cc: Christoph Hellwig <hch@lst.de> >> Cc: Matthew Wilcox <willy@infradead.org> >> Signed-off-by: John Hubbard <jhubbard@nvidia.com> >> --- >> arch/s390/mm/fault.c | 2 +- >> fs/pipe.c | 2 +- >> include/linux/mm.h | 10 +--------- >> mm/gup.c | 21 +++++++++++++++++---- >> 4 files changed, 20 insertions(+), 15 deletions(-) >> >> diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c >> index 212632d57db9..fe1d2c1dbe3b 100644 >> --- a/arch/s390/mm/fault.c >> +++ b/arch/s390/mm/fault.c >> @@ -822,7 +822,7 @@ void do_secure_storage_access(struct pt_regs *regs) >> break; >> case KERNEL_FAULT: >> page = phys_to_page(addr); >> - if (unlikely(!try_get_page(page))) >> + if (unlikely(try_get_compound_head(page, 1) == NULL)) >> break; >> rc = arch_make_page_accessible(page); >> put_page(page); >> diff --git a/fs/pipe.c b/fs/pipe.c >> index 8e6ef62aeb1c..06ba9df37410 100644 >> --- a/fs/pipe.c >> +++ b/fs/pipe.c >> @@ -191,7 +191,7 @@ EXPORT_SYMBOL(generic_pipe_buf_try_steal); >> */ >> bool generic_pipe_buf_get(struct pipe_inode_info *pipe, struct pipe_buffer *buf) >> { >> - return try_get_page(buf->page); >> + return try_get_compound_head(buf->page, 1) != NULL; >> } >> EXPORT_SYMBOL(generic_pipe_buf_get); >> >> diff --git a/include/linux/mm.h b/include/linux/mm.h >> index ce8fc0fd6d6e..cd00d1222235 100644 >> --- a/include/linux/mm.h >> +++ b/include/linux/mm.h >> @@ -1207,15 +1207,7 @@ bool __must_check try_grab_page(struct page *page, unsigned int flags); >> __maybe_unused struct page *try_grab_compound_head(struct page *page, int refs, >> unsigned int flags); >> >> - >> -static inline __must_check bool try_get_page(struct page *page) >> -{ >> - page = compound_head(page); >> - if (WARN_ON_ONCE(page_ref_count(page) <= 0)) >> - return false; >> - page_ref_inc(page); >> - return true; >> -} This is where try_get_page() is removed entirely. thanks,
> On Aug 11, 2021, at 4:07 PM, John Hubbard <jhubbard@nvidia.com> wrote: > > On 8/11/21 1:35 PM, William Kucharski wrote: >> I agree that try_get_page() should probably be removed entirely; is there >> a reason you didn't in v2 of the patch? > > Hi William, > > This patch *does* remove try_get_page() entirely! Look below. I'll reply > inline, below, to show where that happens. Ah, my bad. I was conflating it with try_grab_page() in patch 2/3, which also seems like it should be an inline, but given your explanation re: try_get_compound_head() it makes perfect sense. For the series: Reviewed-by: William Kucharski <william.kucharski@oracle.com>
On Wed, Aug 11, 2021 at 12:05:42AM -0700, John Hubbard wrote: > - if (unlikely(!try_get_page(page))) > + if (unlikely(try_get_compound_head(page, 1) == NULL)) Why not a simple ! instead of the == NULL? Otherwise looks good: Reviewed-by: Christoph Hellwig <hch@lst.de>
On 8/12/21 2:22 AM, Christoph Hellwig wrote: > On Wed, Aug 11, 2021 at 12:05:42AM -0700, John Hubbard wrote: >> - if (unlikely(!try_get_page(page))) >> + if (unlikely(try_get_compound_head(page, 1) == NULL)) > > Why not a simple ! instead of the == NULL? I'll fix those up and post a v4. > > Otherwise looks good: > > Reviewed-by: Christoph Hellwig <hch@lst.de> > Thanks for the reviews! thanks,
diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c index 212632d57db9..fe1d2c1dbe3b 100644 --- a/arch/s390/mm/fault.c +++ b/arch/s390/mm/fault.c @@ -822,7 +822,7 @@ void do_secure_storage_access(struct pt_regs *regs) break; case KERNEL_FAULT: page = phys_to_page(addr); - if (unlikely(!try_get_page(page))) + if (unlikely(try_get_compound_head(page, 1) == NULL)) break; rc = arch_make_page_accessible(page); put_page(page); diff --git a/fs/pipe.c b/fs/pipe.c index 8e6ef62aeb1c..06ba9df37410 100644 --- a/fs/pipe.c +++ b/fs/pipe.c @@ -191,7 +191,7 @@ EXPORT_SYMBOL(generic_pipe_buf_try_steal); */ bool generic_pipe_buf_get(struct pipe_inode_info *pipe, struct pipe_buffer *buf) { - return try_get_page(buf->page); + return try_get_compound_head(buf->page, 1) != NULL; } EXPORT_SYMBOL(generic_pipe_buf_get); diff --git a/include/linux/mm.h b/include/linux/mm.h index ce8fc0fd6d6e..cd00d1222235 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -1207,15 +1207,7 @@ bool __must_check try_grab_page(struct page *page, unsigned int flags); __maybe_unused struct page *try_grab_compound_head(struct page *page, int refs, unsigned int flags); - -static inline __must_check bool try_get_page(struct page *page) -{ - page = compound_head(page); - if (WARN_ON_ONCE(page_ref_count(page) <= 0)) - return false; - page_ref_inc(page); - return true; -} +struct page *try_get_compound_head(struct page *page, int refs); /** * folio_put - Decrement the reference count on a folio. diff --git a/mm/gup.c b/mm/gup.c index 64798d6b5043..c2d19d370c99 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -62,11 +62,24 @@ static void put_page_refs(struct page *page, int refs) put_page(page); } -/* - * Return the compound head page with ref appropriately incremented, - * or NULL if that failed. +/** + * try_get_compound_head() - return the compound head page with refcount + * appropriately incremented, or NULL if that failed. + * + * This handles potential refcount overflow correctly. It also works correctly + * for various lockless get_user_pages()-related callers, due to the use of + * page_cache_add_speculative(). + * + * Even though the name includes "compound_head", this function is still + * appropriate for callers that have a non-compound @page to get. + * + * @page: pointer to page to be gotten + * @refs: the value to add to the page's refcount + * + * Return: head page (with refcount appropriately incremented) for success, or + * NULL upon failure. */ -static inline struct page *try_get_compound_head(struct page *page, int refs) +struct page *try_get_compound_head(struct page *page, int refs) { struct page *head = compound_head(page);
try_get_page() is very similar to try_get_compound_head(), and in fact try_get_page() has fallen a little behind in terms of maintenance: try_get_compound_head() handles speculative page references more thoroughly. There are only two try_get_page() callsites, so just call try_get_compound_head() directly from those, and remove try_get_page() entirely. Also, seeing as how this changes try_get_compound_head() into a non-static function, provide some kerneldoc documentation for it. Cc: Christoph Hellwig <hch@lst.de> Cc: Matthew Wilcox <willy@infradead.org> Signed-off-by: John Hubbard <jhubbard@nvidia.com> --- arch/s390/mm/fault.c | 2 +- fs/pipe.c | 2 +- include/linux/mm.h | 10 +--------- mm/gup.c | 21 +++++++++++++++++---- 4 files changed, 20 insertions(+), 15 deletions(-)