Re: [PATCH v2 3/3] mm/gup: Remove try_get_page(), call try_get_compound_head() directly

[John Hubbard]

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@xxxxxxxxxx> 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@xxxxxx>
> > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
> > ---
> > 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,
--
John Hubbard
NVIDIA


> > +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