linux-kernel - [RFC][PATCH] memcg: shmem swap cache

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Message-Id: <20080702111618.3d7138bd.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Wed, 2 Jul 2008 11:16:18 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	kamezawa.hiroyu@...fujitsu.com
Cc:	Hugh Dickins <hugh@...itas.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Lee Schermerhorn <lee.schermerhorn@...com>,
	Balbir Singh <balbir@...ibm.com>, linux-kernel@...r.kernel.org,
	"kosaki.motohiro@...fujitsu.com" <kosaki.motohiro@...fujitsu.com>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>
Subject: [RFC][PATCH] memcg: shmem swap cache

On Tue, 1 Jul 2008 09:56:45 +0900 (JST)
kamezawa.hiroyu@...fujitsu.com wrote:

> >>   - shmem's page is removed by mem_cgroup_uncharge_cache_page(), So,
> >>     shmem's swapcache is not charged.
> >
> >Ah, that's interesting: I'd assumed you'd changed that in your
> >no-refcount patches, and had been surprised not to notice a slowdown
> >(waiting for swap to be written and freed before coming under limit).
> >Now you want to make them wait: not entirely an improvement,
> >but I see your point.
> >
> To do this is (maybe) a few line patch. I'll CC you if I wrote some.
> 
maybe like this. how do you feel ?
I'll stack this on my queue and test for a while.

==
SwapCache handling fix.

memcg failed to handle shmem's swapcache. This patch tries to fix it.

After this:

Any page marked as SwapCache is not uncharged.
(until delelte_from_swap_cache() delete the flag.)

To check a page is alive shmem-page-cache or not we use
 page->mapping && !PageAnon(page) instead of pc->flags & PAGE_CGROUP_FLAG_CACHE.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>

Index: test-2.6.26-rc5-mm3++/mm/memcontrol.c
===================================================================
--- test-2.6.26-rc5-mm3++.orig/mm/memcontrol.c	2008-07-02 09:29:52.000000000 +0900
+++ test-2.6.26-rc5-mm3++/mm/memcontrol.c	2008-07-02 10:58:15.000000000 +0900
@@ -685,11 +685,45 @@
 
 	VM_BUG_ON(pc->page != page);
 
-	if ((ctype == MEM_CGROUP_CHARGE_TYPE_MAPPED)
-	    && ((pc->flags & PAGE_CGROUP_FLAG_CACHE)
-		|| page_mapped(page)
-		|| PageSwapCache(page)))
+	/*
+	 * File Cache
+	 * If called with MEM_CGROUP_CHARGE_TYPE_MAPPED, check page->mapping.
+	 * add_to_page_cache() .... charged before inserting radix-tree.
+	 * remove_from_page_cache() .... uncharged at removing from radix-tree.
+	 * page->mapping && !PageAnon(page) catches file cache.
+	 *
+	 * Anon/Shmem.....We check PageSwapCache(page).
+	 * Anon .... charged before mapped.
+	 * Shmem .... charged at add_to_page_cache() as usual File Cache.
+	 *
+	 * This page will be finally uncharged when removed from swap-cache
+	 *
+	 * we treat 2 cases here.
+	 * A. anonymous page  B. shmem.
+	 * We never uncharge if page is marked as SwapCache.
+	 * add_to_swap_cache() have nothing to do with charge/uncharge.
+	 * SwapCache flag is deleted before delete_from_swap_cache() calls this
+	 *
+	 * shmem's behavior is following. (see shmem.c/swap_state.c also)
+	 * at swap-out:
+	 * 	0. add_to_page_cache()//charged at page creation.
+	 * 	1. add_to_swap_cache() (marked as SwapCache)
+	 *	2. remove_from_page_cache().  (calls this.)
+	 *	(finally) delete_from_swap_cache(). (calls this.)
+	 * at swap-in:
+	 * 	3. add_to_swap_cache() (no charge here.)
+	 * 	4. add_to_page_cache() (charged here.)
+	 * 	5. delete_from_swap_cache() (calls this.)
+	 * PageSwapCache(page) catches "2".
+	 * page->mapping && !PageAnon() catches "5" and avoid uncharging.
+	 */
+	if (PageSwapCache(page))
 		goto unlock;
+	/* called from unmap or delete_from_swap_cache() */
+	if ((ctype == MEM_CGROUP_CHARGE_TYPE_MAPPED)
+		&& (page_mapped(page)
+		    || (page->mapping && !PageAnon(page))))/* alive cache ? */
+			goto unlock;
 
 	mz = page_cgroup_zoneinfo(pc);
 	spin_lock_irqsave(&mz->lru_lock, flags);

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