linux-kernel - Re: [PATCH 03/24] mm: use __SetPageSwapBacked and don't ClearPageSwapBacked

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Message-ID: <20150225105315.GK23372@suse.de>
Date:	Wed, 25 Feb 2015 10:53:15 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Ning Qu <quning@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 03/24] mm: use __SetPageSwapBacked and don't
 ClearPageSwapBacked

On Fri, Feb 20, 2015 at 07:56:15PM -0800, Hugh Dickins wrote:
> Commit 07a427884348 ("mm: shmem: avoid atomic operation during
> shmem_getpage_gfp") rightly replaced one instance of SetPageSwapBacked
> by __SetPageSwapBacked, pointing out that the newly allocated page is
> not yet visible to other users (except speculative get_page_unless_zero-
> ers, who may not update page flags before their further checks).
> 
> That was part of a series in which Mel was focused on tmpfs profiles:
> but almost all SetPageSwapBacked uses can be so optimized, with the
> same justification.  And remove the ClearPageSwapBacked from
> read_swap_cache_async()'s and zswap_get_swap_cache_page()'s error
> paths: it's not an error to free a page with PG_swapbacked set.
> 
> (There's probably scope for further __SetPageFlags in other places,
> but SwapBacked is the one I'm interested in at the moment.)
> 
> Signed-off-by: Hugh Dickins <hughd@...gle.com>
> ---
>  mm/migrate.c    |    6 +++---
>  mm/rmap.c       |    2 +-
>  mm/shmem.c      |    4 ++--
>  mm/swap_state.c |    3 +--
>  mm/zswap.c      |    3 +--
>  5 files changed, 8 insertions(+), 10 deletions(-)
> 
> <SNIP>
> --- thpfs.orig/mm/shmem.c	2015-02-08 18:54:22.000000000 -0800
> +++ thpfs/mm/shmem.c	2015-02-20 19:33:35.676074594 -0800
> @@ -987,8 +987,8 @@ static int shmem_replace_page(struct pag
>  	flush_dcache_page(newpage);
>  
>  	__set_page_locked(newpage);
> +	__SetPageSwapBacked(newpage);
>  	SetPageUptodate(newpage);
> -	SetPageSwapBacked(newpage);
>  	set_page_private(newpage, swap_index);
>  	SetPageSwapCache(newpage);
>  

It's clear why you did this but ...

> @@ -1177,8 +1177,8 @@ repeat:
>  			goto decused;
>  		}
>  
> -		__SetPageSwapBacked(page);
>  		__set_page_locked(page);
> +		__SetPageSwapBacked(page);
>  		if (sgp == SGP_WRITE)
>  			__SetPageReferenced(page);
>  

It's less clear why this was necessary. I don't think it causes any
problems though so

Reviewed-by: Mel Gorman <mgorman@...e.de>

-- 
Mel Gorman
SUSE Labs
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