linux-kernel - [BUGFIX][PATCH] fix race in file

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Date:	Wed, 24 Mar 2010 15:43:24 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	"linux-mm@...ck.org" <linux-mm@...ck.org>
Cc:	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	arighi@...eler.com
Subject: [BUGFIX][PATCH] fix race in file_mapped accounting in memcg

A fix for race in file_mapped statistics. I noticed this race while discussing
Andrea's dirty accounting patch series. 
At the end of discusstion, I said "please don't touch file mapped". So, this bugfix
should be posted as an independent patch.
Tested on the latest mmotm.

Thanks,
-Kame

==
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>

Now, memcg's FILE_MAPPED accounting has following race with
move_account (happens at rmdir()).

    increment page->mapcount (rmap.c)
    mem_cgroup_update_file_mapped()           move_account()
					      lock_page_cgroup()
					      check page_mapped() if
					      page_mapped(page)>1 {
						FILE_MAPPED -1 from old memcg
						FILE_MAPPED +1 to old memcg
					      }
					      .....
					      overwrite pc->mem_cgroup
					      unlock_page_cgroup()
    lock_page_cgroup()
    FILE_MAPPED + 1 to pc->mem_cgroup
    unlock_page_cgroup()

Then,
	old memcg (-1 file mapped)
	new memcg (+2 file mapped)


This happens because move_account see page_mapped() which is not guarded by
lock_page_cgroup(). This patch adds FILE_MAPPED flag to page_cgroup and
move account information based on it. Now, all checks are synchronous with
lock_page_cgroup().


Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
---
 include/linux/page_cgroup.h |    6 ++++++
 mm/memcontrol.c             |   18 +++++++++---------
 2 files changed, 15 insertions(+), 9 deletions(-)

Index: mmotm-2.6.34-Mar23/include/linux/page_cgroup.h
===================================================================
--- mmotm-2.6.34-Mar23.orig/include/linux/page_cgroup.h
+++ mmotm-2.6.34-Mar23/include/linux/page_cgroup.h
@@ -39,6 +39,7 @@ enum {
 	PCG_CACHE, /* charged as cache */
 	PCG_USED, /* this object is in use. */
 	PCG_ACCT_LRU, /* page has been accounted for */
+	PCG_FILE_MAPPED, /* page is accounted as "mapped" */
 };
 
 #define TESTPCGFLAG(uname, lname)			\
@@ -73,6 +74,11 @@ CLEARPCGFLAG(AcctLRU, ACCT_LRU)
 TESTPCGFLAG(AcctLRU, ACCT_LRU)
 TESTCLEARPCGFLAG(AcctLRU, ACCT_LRU)
 
+
+SETPCGFLAG(FileMapped, FILE_MAPPED)
+CLEARPCGFLAG(FileMapped, FILE_MAPPED)
+TESTPCGFLAG(FileMapped, FILE_MAPPED)
+
 static inline int page_cgroup_nid(struct page_cgroup *pc)
 {
 	return page_to_nid(pc->page);
Index: mmotm-2.6.34-Mar23/mm/memcontrol.c
===================================================================
--- mmotm-2.6.34-Mar23.orig/mm/memcontrol.c
+++ mmotm-2.6.34-Mar23/mm/memcontrol.c
@@ -1430,16 +1430,19 @@ void mem_cgroup_update_file_mapped(struc
 
 	lock_page_cgroup(pc);
 	mem = pc->mem_cgroup;
-	if (!mem)
-		goto done;
-
-	if (!PageCgroupUsed(pc))
+	if (!mem || !PageCgroupUsed(pc))
 		goto done;
 
 	/*
 	 * Preemption is already disabled. We can use __this_cpu_xxx
 	 */
-	__this_cpu_add(mem->stat->count[MEM_CGROUP_STAT_FILE_MAPPED], val);
+	if (val > 0) {
+		__this_cpu_inc(mem->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
+		SetPageCgroupFileMapped(pc);
+	} else {
+		__this_cpu_dec(mem->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
+		ClearPageCgroupFileMapped(pc);
+	}
 
 done:
 	unlock_page_cgroup(pc);
@@ -1872,16 +1875,13 @@ static void __mem_cgroup_commit_charge(s
 static void __mem_cgroup_move_account(struct page_cgroup *pc,
 	struct mem_cgroup *from, struct mem_cgroup *to, bool uncharge)
 {
-	struct page *page;
-
 	VM_BUG_ON(from == to);
 	VM_BUG_ON(PageLRU(pc->page));
 	VM_BUG_ON(!PageCgroupLocked(pc));
 	VM_BUG_ON(!PageCgroupUsed(pc));
 	VM_BUG_ON(pc->mem_cgroup != from);
 
-	page = pc->page;
-	if (page_mapped(page) && !PageAnon(page)) {
+	if (PageCgroupFileMapped(pc)) {
 		/* Update mapped_file data for mem_cgroup */
 		preempt_disable();
 		__this_cpu_dec(from->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);

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