linux-kernel - Re: [PATCH 1/4] mm: m(un)lock avoid ZERO

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Date:	Wed, 16 Sep 2009 09:08:15 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Hugh Dickins <hugh.dickins@...cali.co.uk>
Cc:	kosaki.motohiro@...fujitsu.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>, Rik van Riel <riel@...hat.com>,
	Mel Gorman <mel@....ul.ie>,
	Minchan Kim <minchan.kim@...il.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 1/4] mm: m(un)lock avoid ZERO_PAGE

> I'm still reluctant to clutter __get_user_pages() with another flag,
> just to avoid touching ZERO_PAGE count in mlock(); though we can add
> that later if it shows up as an issue in practice.
> 
> But when mlocking, we can test page->mapping slightly earlier, to avoid
> the potentially bouncy rescheduling of lock_page on ZERO_PAGE - mlock
> didn't lock_page in olden ZERO_PAGE days, so we might have regressed.
> 
> And when munlocking, it turns out that FOLL_DUMP coincidentally does
> what's needed to avoid all updates to ZERO_PAGE, so use that here also.
> Plus add comment suggested by KAMEZAWA Hiroyuki.
> 
> Signed-off-by: Hugh Dickins <hugh.dickins@...cali.co.uk>
> ---
> 
>  mm/mlock.c |   49 ++++++++++++++++++++++++++++++++++++-------------
>  1 file changed, 36 insertions(+), 13 deletions(-)
> 
> --- mm0/mm/mlock.c	2009-09-14 16:34:37.000000000 +0100
> +++ mm1/mm/mlock.c	2009-09-15 17:32:03.000000000 +0100
> @@ -198,17 +198,26 @@ static long __mlock_vma_pages_range(stru
>  		for (i = 0; i < ret; i++) {
>  			struct page *page = pages[i];
>  
> -			lock_page(page);
> -			/*
> -			 * Because we lock page here and migration is blocked
> -			 * by the elevated reference, we need only check for
> -			 * file-cache page truncation.  This page->mapping
> -			 * check also neatly skips over the ZERO_PAGE(),
> -			 * though if that's common we'd prefer not to lock it.
> -			 */
> -			if (page->mapping)
> -				mlock_vma_page(page);
> -			unlock_page(page);
> +			if (page->mapping) {
> +				/*
> +				 * That preliminary check is mainly to avoid
> +				 * the pointless overhead of lock_page on the
> +				 * ZERO_PAGE: which might bounce very badly if
> +				 * there is contention.  However, we're still
> +				 * dirtying its cacheline with get/put_page:
> +				 * we'll add another __get_user_pages flag to
> +				 * avoid it if that case turns out to matter.
> +				 */
> +				lock_page(page);
> +				/*
> +				 * Because we lock page here and migration is
> +				 * blocked by the elevated reference, we need
> +				 * only check for file-cache page truncation.
> +				 */
> +				if (page->mapping)
> +					mlock_vma_page(page);
> +				unlock_page(page);
> +			}
>  			put_page(page);	/* ref from get_user_pages() */
>  		}

Yes, I have similar patch :-)
	Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>




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