linux-kernel - Re: [PATCH v2 17/28] gup: Add gup_put

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Message-ID: <725ee15c-e2c0-ca74-995f-a6f8b5196f62@nvidia.com>
Date:   Mon, 10 Jan 2022 22:44:30 -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 17/28] gup: Add gup_put_folio()

On 1/9/22 20:23, Matthew Wilcox (Oracle) wrote:
> Convert put_compound_head() to gup_put_folio() and hpage_pincount_sub()
> to folio_pincount_sub().  This removes the last call to put_page_refs(),
> so delete it.  Add a temporary put_compound_head() wrapper which will
> be deleted by the end of this series.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> ---
>   mm/gup.c | 42 ++++++++++++++----------------------------
>   1 file changed, 14 insertions(+), 28 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 9e581201d679..719252fa0402 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -45,34 +45,15 @@ static void folio_pincount_add(struct folio *folio, int refs)
>   		folio_ref_add(folio, refs * (GUP_PIN_COUNTING_BIAS - 1));
>   }
>   
> -static int page_pincount_sub(struct page *page, int refs)
> +static int folio_pincount_sub(struct folio *folio, int refs)

I just noticed that this return value was a little hard to reason about, which
prompted me to notice that I'd skipped reviewing patch 10. So I went back
and added a note there. :)

Anyway, this one looks good,

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


thanks,
-- 
John Hubbard
NVIDIA

>   {
> -	VM_BUG_ON_PAGE(page != compound_head(page), page);
> -
> -	if (PageHead(page))
> -		atomic_sub(refs, compound_pincount_ptr(page));
> +	if (folio_test_large(folio))
> +		atomic_sub(refs, folio_pincount_ptr(folio));
>   	else
>   		refs *= GUP_PIN_COUNTING_BIAS;
>   	return refs;
>   }
>   
> -/* Equivalent to calling put_page() @refs times. */
> -static void put_page_refs(struct page *page, int refs)
> -{
> -#ifdef CONFIG_DEBUG_VM
> -	if (VM_WARN_ON_ONCE_PAGE(page_ref_count(page) < refs, page))
> -		return;
> -#endif
> -
> -	/*
> -	 * Calling put_page() for each ref is unnecessarily slow. Only the last
> -	 * ref needs a put_page().
> -	 */
> -	if (refs > 1)
> -		page_ref_sub(page, refs - 1);
> -	put_page(page);
> -}
> -
>   /*
>    * Return the folio with ref appropriately incremented,
>    * or NULL if that failed.
> @@ -171,15 +152,20 @@ struct page *try_grab_compound_head(struct page *page,
>   	return &try_grab_folio(page, refs, flags)->page;
>   }
>   
> -static void put_compound_head(struct page *page, int refs, unsigned int flags)
> +static void gup_put_folio(struct folio *folio, int refs, unsigned int flags)
>   {
>   	if (flags & FOLL_PIN) {
> -		mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_RELEASED,
> -				    refs);
> -		refs = page_pincount_sub(page, refs);
> +		node_stat_mod_folio(folio, NR_FOLL_PIN_RELEASED, refs);
> +		refs = folio_pincount_sub(folio, refs);
>   	}
>   
> -	put_page_refs(page, refs);
> +	folio_put_refs(folio, refs);
> +}
> +
> +static void put_compound_head(struct page *page, int refs, unsigned int flags)
> +{
> +	VM_BUG_ON_PAGE(PageTail(page), page);
> +	gup_put_folio((struct folio *)page, refs, flags);
>   }
>   
>   /**
> @@ -220,7 +206,7 @@ bool __must_check try_grab_page(struct page *page, unsigned int flags)
>    */
>   void unpin_user_page(struct page *page)
>   {
> -	put_compound_head(compound_head(page), 1, FOLL_PIN);
> +	gup_put_folio(page_folio(page), 1, FOLL_PIN);
>   }
>   EXPORT_SYMBOL(unpin_user_page);
>