linux-kernel - Re: [PATCH v2 08/28] gup: Handle page split race more efficiently

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Message-ID: <c7ca4374-4389-8bb5-ad0d-8d2e8c0784e2@nvidia.com>
Date:   Mon, 10 Jan 2022 19:30:25 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>, linux-mm@...ck.org
Cc:     Christoph Hellwig <hch@...radead.org>,
        William Kucharski <william.kucharski@...cle.com>,
        linux-kernel@...r.kernel.org, Jason Gunthorpe <jgg@...pe.ca>
Subject: Re: [PATCH v2 08/28] gup: Handle page split race more efficiently

On 1/9/22 20:23, Matthew Wilcox (Oracle) wrote:
> If we hit the page split race, the current code returns NULL which will
> presumably trigger a retry under the mmap_lock.  This isn't necessary;
> we can just retry the compound_head() lookup.  This is a very minor
> optimisation of an unlikely path, but conceptually it matches (eg)
> the page cache RCU-protected lookup.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> ---
>   mm/gup.c | 7 +++++--
>   1 file changed, 5 insertions(+), 2 deletions(-)


Reviewed-by: John Hubbard <jhubbard@...dia.com>


thanks,
-- 
John Hubbard
NVIDIA

> 
> diff --git a/mm/gup.c b/mm/gup.c
> index afb638a30e44..dbb1b54d0def 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -68,7 +68,10 @@ static void put_page_refs(struct page *page, int refs)
>    */
>   static inline struct page *try_get_compound_head(struct page *page, int refs)
>   {
> -	struct page *head = compound_head(page);
> +	struct page *head;
> +
> +retry:
> +	head = compound_head(page);
>   
>   	if (WARN_ON_ONCE(page_ref_count(head) < 0))
>   		return NULL;
> @@ -86,7 +89,7 @@ static inline struct page *try_get_compound_head(struct page *page, int refs)
>   	 */
>   	if (unlikely(compound_head(page) != head)) {
>   		put_page_refs(head, refs);
> -		return NULL;
> +		goto retry;
>   	}
>   
>   	return head;