linux-kernel - Re: [PATCH] mm: munlock use mapcount to avoid terrible overhead

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Date:	Tue, 18 Oct 2011 17:14:53 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Michel Lespinasse <walken@...gle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: munlock use mapcount to avoid terrible overhead

On Tue, 18 Oct 2011 17:02:56 -0700 (PDT)
Hugh Dickins <hughd@...gle.com> wrote:

> A process spent 30 minutes exiting, just munlocking the pages of a large
> anonymous area that had been alternately mprotected into page-sized vmas:
> for every single page there's an anon_vma walk through all the other
> little vmas to find the right one.
> 
> A general fix to that would be a lot more complicated (use prio_tree on
> anon_vma?), but there's one very simple thing we can do to speed up the
> common case: if a page to be munlocked is mapped only once, then it is
> our vma that it is mapped into, and there's no need whatever to walk
> through all the others.
> 
> Okay, there is a very remote race in munlock_vma_pages_range(), if
> between its follow_page() and lock_page(), another process were to
> munlock the same page, then page reclaim remove it from our vma, then
> another process mlock it again.  We would find it with page_mapcount
> 1, yet it's still mlocked in another process.  But never mind, that's
> much less likely than the down_read_trylock() failure which munlocking
> already tolerates (in try_to_unmap_one()): in due course page reclaim
> will discover and move the page to unevictable instead.
>     

And how long did the test case take with the patch applied?

> ---
>  mm/mlock.c |    5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> --- 3.1-rc10/mm/mlock.c	2011-07-21 19:17:23.000000000 -0700
> +++ linux/mm/mlock.c	2011-10-06 12:47:54.670436979 -0700
> @@ -110,7 +110,10 @@ void munlock_vma_page(struct page *page)
>  	if (TestClearPageMlocked(page)) {
>  		dec_zone_page_state(page, NR_MLOCK);
>  		if (!isolate_lru_page(page)) {
> -			int ret = try_to_munlock(page);
> +			int ret = SWAP_AGAIN;
> +
> +			if (page_mapcount(page) > 1)
> +				ret = try_to_munlock(page);
>  			/*
>  			 * did try_to_unlock() succeed or punt?
>  			 */

tsk.

--- a/mm/mlock.c~mm-munlock-use-mapcount-to-avoid-terrible-overhead-fix
+++ a/mm/mlock.c
@@ -112,6 +112,11 @@ void munlock_vma_page(struct page *page)
 		if (!isolate_lru_page(page)) {
 			int ret = SWAP_AGAIN;
 
+			/*
+			 * Optimization: if the page was mapped just once,
+			 * that's our mapping and we don't need to check all the
+			 * other vmas.
+			 */
 			if (page_mapcount(page) > 1)
 				ret = try_to_munlock(page);
 			/*
_

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