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@xxxxxxxxxxxxxx>
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@xxxxxxxxxxxxxx>
---
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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