linux-kernel - Re: [PATCH 10/75] mm/gup: Remove hpage_pincount

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Message-ID: <0c128b92-7dda-07be-f6e0-80be0b91d7cc@nvidia.com>
Date:   Fri, 4 Feb 2022 13:29:55 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>, linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/75] mm/gup: Remove hpage_pincount_add()

On 2/4/22 11:57, Matthew Wilcox (Oracle) wrote:
> It's clearer to call atomic_add() in the callers; the assertions clearly
> can't fire there because they're part of the condition for calling
> atomic_add().
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> ---
>   mm/gup.c | 33 +++++++++++----------------------
>   1 file changed, 11 insertions(+), 22 deletions(-)

Looks nice.

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

thanks,
-- 
John Hubbard
NVIDIA

> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 923a0d44203c..60168a09d52a 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -29,14 +29,6 @@ struct follow_page_context {
>   	unsigned int page_mask;
>   };
>   
> -static void hpage_pincount_add(struct page *page, int refs)
> -{
> -	VM_BUG_ON_PAGE(!hpage_pincount_available(page), page);
> -	VM_BUG_ON_PAGE(page != compound_head(page), page);
> -
> -	atomic_add(refs, compound_pincount_ptr(page));
> -}
> -
>   static void hpage_pincount_sub(struct page *page, int refs)
>   {
>   	VM_BUG_ON_PAGE(!hpage_pincount_available(page), page);
> @@ -151,17 +143,17 @@ __maybe_unused struct page *try_grab_compound_head(struct page *page,
>   			return NULL;
>   
>   		/*
> -		 * When pinning a compound page of order > 1 (which is what
> -		 * hpage_pincount_available() checks for), use an exact count to
> -		 * track it, via hpage_pincount_add/_sub().
> +		 * When pinning a compound page of order > 1 (which is
> +		 * what hpage_pincount_available() checks for), use an
> +		 * exact count to track it.
>   		 *
> -		 * However, be sure to *also* increment the normal page refcount
> -		 * field at least once, so that the page really is pinned.
> -		 * That's why the refcount from the earlier
> +		 * However, be sure to *also* increment the normal page
> +		 * refcount field at least once, so that the page really
> +		 * is pinned.  That's why the refcount from the earlier
>   		 * try_get_compound_head() is left intact.
>   		 */
>   		if (hpage_pincount_available(page))
> -			hpage_pincount_add(page, refs);
> +			atomic_add(refs, compound_pincount_ptr(page));
>   		else
>   			page_ref_add(page, refs * (GUP_PIN_COUNTING_BIAS - 1));
>   
> @@ -216,22 +208,19 @@ bool __must_check try_grab_page(struct page *page, unsigned int flags)
>   	if (flags & FOLL_GET)
>   		return try_get_page(page);
>   	else if (flags & FOLL_PIN) {
> -		int refs = 1;
> -
>   		page = compound_head(page);
>   
>   		if (WARN_ON_ONCE(page_ref_count(page) <= 0))
>   			return false;
>   
>   		/*
> -		 * Similar to try_grab_compound_head(): even if using the
> -		 * hpage_pincount_add/_sub() routines, be sure to
> -		 * *also* increment the normal page refcount field at least
> -		 * once, so that the page really is pinned.
> +		 * Similar to try_grab_compound_head(): be sure to *also*
> +		 * increment the normal page refcount field at least once,
> +		 * so that the page really is pinned.
>   		 */
>   		if (hpage_pincount_available(page)) {
>   			page_ref_add(page, 1);
> -			hpage_pincount_add(page, 1);
> +			atomic_add(1, compound_pincount_ptr(page));
>   		} else {
>   			page_ref_add(page, GUP_PIN_COUNTING_BIAS);
>   		}