linux-kernel - Re: [PATCH v2 3/3] mm/gup: Remove try_get_page(), call try_get_compound

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Date:   Wed, 11 Aug 2021 20:35:05 +0000
From:   William Kucharski <william.kucharski@...cle.com>
To:     John Hubbard <jhubbard@...dia.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v2 3/3] mm/gup: Remove try_get_page(), call
 try_get_compound_head() directly

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@...dia.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@....de>
> Cc: Matthew Wilcox <willy@...radead.org>
> Signed-off-by: John Hubbard <jhubbard@...dia.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
> 
>