[RFC][-mm] [1/7] shmem swapcache fix

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

SwapCache handling fix.

shmem's swapcache behavior is a little different from anonymous's one and
memcg failed to handle it. This patch tries to fix it.

After this:

Any page marked as SwapCache is not uncharged. (delelte_from_swap_cache()
delete the flag.) Because SwapCache is accounted, this is not a good change
for performance of shmem/tmpfs under memcg. (But meory was leaked.)
We need additional fix of background-job, dirty_ratio, etc..

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@jp.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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