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Commit 8725d541 authored by KAMEZAWA Hiroyuki's avatar KAMEZAWA Hiroyuki Committed by Linus Torvalds
Browse files

memcg: fix race in file_mapped accounting 



Presently, 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: default avatarKAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reviewed-by: default avatarBalbir Singh <balbir@in.ibm.com>
Reviewed-by: default avatarDaisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Andrea Righi <arighi@develer.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 116354d1

include/linux/page_cgroup.h

+6 −0
Original line number Diff line number Diff line
@@ -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);

mm/memcontrol.c

+9 −9
Original line number Diff line number Diff line
@@ -1359,16 +1359,19 @@ void mem_cgroup_update_file_mapped(struct page *page, int val) 


	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);
@@ -1801,16 +1804,13 @@ static void __mem_cgroup_commit_charge(struct mem_cgroup *mem, 
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]);