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memcg: use new logic for page stat accounting

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Now, page-stat-per-memcg is recorded into per page_cgroup flag by
duplicating page's status into the flag.  The reason is that memcg has a
feature to move a page from a group to another group and we have race
between "move" and "page stat accounting",

Under current logic, assume CPU-A and CPU-B.  CPU-A does "move" and CPU-B
does "page stat accounting".

When CPU-A goes 1st,

            CPU-A                           CPU-B
                                    update "struct page" info.
    move_lock_mem_cgroup(memcg)
    see pc->flags
    copy page stat to new group
    overwrite pc->mem_cgroup.
    move_unlock_mem_cgroup(memcg)
                                    move_lock_mem_cgroup(mem)
                                    set pc->flags
                                    update page stat accounting
                                    move_unlock_mem_cgroup(mem)

stat accounting is guarded by move_lock_mem_cgroup() and "move" logic
(CPU-A) doesn't see changes in "struct page" information.

But it's costly to have the same information both in 'struct page' and
'struct page_cgroup'.  And, there is a potential problem.

For example, assume we have PG_dirty accounting in memcg.
PG_..is a flag for struct page.
PCG_ is a flag for struct page_cgroup.
(This is just an example. The same problem can be found in any
 kind of page stat accounting.)

	  CPU-A                               CPU-B
      TestSet PG_dirty
      (delay)                        TestClear PG_dirty
                                     if (TestClear(PCG_dirty))
                                          memcg->nr_dirty--
      if (TestSet(PCG_dirty))
          memcg->nr_dirty++

Here, memcg->nr_dirty = +1, this is wrong.  This race was reported by Greg
Thelen <gthelen@google.com>.  Now, only FILE_MAPPED is supported but
fortunately, it's serialized by page table lock and this is not real bug,
_now_,

If this potential problem is caused by having duplicated information in
struct page and struct page_cgroup, we may be able to fix this by using
original 'struct page' information.  But we'll have a problem in "move
account"

Assume we use only PG_dirty.

         CPU-A                   CPU-B
    TestSet PG_dirty
    (delay)                    move_lock_mem_cgroup()
                               if (PageDirty(page))
                                      new_memcg->nr_dirty++
                               pc->mem_cgroup = new_memcg;
                               move_unlock_mem_cgroup()
    move_lock_mem_cgroup()
    memcg = pc->mem_cgroup
    new_memcg->nr_dirty++

accounting information may be double-counted.  This was original reason to
have PCG_xxx flags but it seems PCG_xxx has another problem.

I think we need a bigger lock as

     move_lock_mem_cgroup(page)
     TestSetPageDirty(page)
     update page stats (without any checks)
     move_unlock_mem_cgroup(page)

This fixes both of problems and we don't have to duplicate page flag into
page_cgroup.  Please note: move_lock_mem_cgroup() is held only when there
are possibility of "account move" under the system.  So, in most path,
status update will go without atomic locks.

This patch introduces mem_cgroup_begin_update_page_stat() and
mem_cgroup_end_update_page_stat() both should be called at modifying
'struct page' information if memcg takes care of it.  as

     mem_cgroup_begin_update_page_stat()
     modify page information
     mem_cgroup_update_page_stat()
     => never check any 'struct page' info, just update counters.
     mem_cgroup_end_update_page_stat().

This patch is slow because we need to call begin_update_page_stat()/
end_update_page_stat() regardless of accounted will be changed or not.  A
following patch adds an easy optimization and reduces the cost.

[akpm@linux-foundation.org: s/lock/locked/]
[hughd@google.com: fix deadlock by avoiding stat lock when anon]
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Greg Thelen <gthelen@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Ying Han <yinghan@google.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
KAMEZAWA Hiroyuki 2012-03-21 16:34:25 -07:00 committed by Linus Torvalds
parent 312734c04e
commit 89c06bd52f
3 changed files with 101 additions and 24 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

35
include/linux/memcontrol.h
View file

@ -141,6 +141,31 @@ static inline bool mem_cgroup_disabled(void)
return false;
}
void __mem_cgroup_begin_update_page_stat(struct page *page, bool *locked,
unsigned long *flags);
static inline void mem_cgroup_begin_update_page_stat(struct page *page,
bool *locked, unsigned long *flags)
{
if (mem_cgroup_disabled())
return;
rcu_read_lock();
*locked = false;
return __mem_cgroup_begin_update_page_stat(page, locked, flags);
}
void __mem_cgroup_end_update_page_stat(struct page *page,
unsigned long *flags);
static inline void mem_cgroup_end_update_page_stat(struct page *page,
bool *locked, unsigned long *flags)
{
if (mem_cgroup_disabled())
return;
if (*locked)
__mem_cgroup_end_update_page_stat(page, flags);
rcu_read_unlock();
}
void mem_cgroup_update_page_stat(struct page *page,
enum mem_cgroup_page_stat_item idx,
int val);
@ -341,6 +366,16 @@ mem_cgroup_print_oom_info(struct mem_cgroup *memcg, struct task_struct *p)
{
}
static inline void mem_cgroup_begin_update_page_stat(struct page *page,
bool *locked, unsigned long *flags)
{
}
static inline void mem_cgroup_end_update_page_stat(struct page *page,
bool *locked, unsigned long *flags)
{
}
static inline void mem_cgroup_inc_page_stat(struct page *page,
enum mem_cgroup_page_stat_item idx)
{

62
mm/memcontrol.c
View file

@ -1910,32 +1910,59 @@ bool mem_cgroup_handle_oom(struct mem_cgroup *memcg, gfp_t mask, int order)
* If there is, we take a lock.
*/
void __mem_cgroup_begin_update_page_stat(struct page *page,
bool *locked, unsigned long *flags)
{
struct mem_cgroup *memcg;
struct page_cgroup *pc;
pc = lookup_page_cgroup(page);
again:
memcg = pc->mem_cgroup;
if (unlikely(!memcg || !PageCgroupUsed(pc)))
return;
/*
* If this memory cgroup is not under account moving, we don't
* need to take move_lock_page_cgroup(). Because we already hold
* rcu_read_lock(), any calls to move_account will be delayed until
* rcu_read_unlock() if mem_cgroup_stealed() == true.
*/
if (!mem_cgroup_stealed(memcg))
return;
move_lock_mem_cgroup(memcg, flags);
if (memcg != pc->mem_cgroup || !PageCgroupUsed(pc)) {
move_unlock_mem_cgroup(memcg, flags);
goto again;
}
*locked = true;
}
void __mem_cgroup_end_update_page_stat(struct page *page, unsigned long *flags)
{
struct page_cgroup *pc = lookup_page_cgroup(page);
/*
* It's guaranteed that pc->mem_cgroup never changes while
* lock is held because a routine modifies pc->mem_cgroup
* should take move_lock_page_cgroup().
*/
move_unlock_mem_cgroup(pc->mem_cgroup, flags);
}
void mem_cgroup_update_page_stat(struct page *page,
enum mem_cgroup_page_stat_item idx, int val)
{
struct mem_cgroup *memcg;
struct page_cgroup *pc = lookup_page_cgroup(page);
bool need_unlock = false;
unsigned long uninitialized_var(flags);
if (mem_cgroup_disabled())
return;
again:
rcu_read_lock();
memcg = pc->mem_cgroup;
if (unlikely(!memcg || !PageCgroupUsed(pc)))
goto out;
/* pc->mem_cgroup is unstable ? */
if (unlikely(mem_cgroup_stealed(memcg))) {
/* take a lock against to access pc->mem_cgroup */
move_lock_mem_cgroup(memcg, &flags);
if (memcg != pc->mem_cgroup || !PageCgroupUsed(pc)) {
move_unlock_mem_cgroup(memcg, &flags);
rcu_read_unlock();
goto again;
}
need_unlock = true;
}
return;
switch (idx) {
case MEMCG_NR_FILE_MAPPED:
@ -1950,11 +1977,6 @@ void mem_cgroup_update_page_stat(struct page *page,
}
this_cpu_add(memcg->stat->count[idx], val);
out:
if (unlikely(need_unlock))
move_unlock_mem_cgroup(memcg, &flags);
rcu_read_unlock();
}
/*

28
mm/rmap.c
View file

@ -1148,10 +1148,15 @@ void page_add_new_anon_rmap(struct page *page,
*/
void page_add_file_rmap(struct page *page)
{
bool locked;
unsigned long flags;
mem_cgroup_begin_update_page_stat(page, &locked, &flags);
if (atomic_inc_and_test(&page->_mapcount)) {
__inc_zone_page_state(page, NR_FILE_MAPPED);
mem_cgroup_inc_page_stat(page, MEMCG_NR_FILE_MAPPED);
}
mem_cgroup_end_update_page_stat(page, &locked, &flags);
}
/**
@ -1162,9 +1167,21 @@ void page_add_file_rmap(struct page *page)
*/
void page_remove_rmap(struct page *page)
{
bool anon = PageAnon(page);
bool locked;
unsigned long flags;
/*
* The anon case has no mem_cgroup page_stat to update; but may
* uncharge_page() below, where the lock ordering can deadlock if
* we hold the lock against page_stat move: so avoid it on anon.
*/
if (!anon)
mem_cgroup_begin_update_page_stat(page, &locked, &flags);
/* page still mapped by someone else? */
if (!atomic_add_negative(-1, &page->_mapcount))
return;
goto out;
/*
* Now that the last pte has gone, s390 must transfer dirty
@ -1173,7 +1190,7 @@ void page_remove_rmap(struct page *page)
* not if it's in swapcache - there might be another pte slot
* containing the swap entry, but page not yet written to swap.
*/
if ((!PageAnon(page) || PageSwapCache(page)) &&
if ((!anon || PageSwapCache(page)) &&
page_test_and_clear_dirty(page_to_pfn(page), 1))
set_page_dirty(page);
/*
@ -1181,8 +1198,8 @@ void page_remove_rmap(struct page *page)
* and not charged by memcg for now.
*/
if (unlikely(PageHuge(page)))
return;
if (PageAnon(page)) {
goto out;
if (anon) {
mem_cgroup_uncharge_page(page);
if (!PageTransHuge(page))
__dec_zone_page_state(page, NR_ANON_PAGES);
@ -1202,6 +1219,9 @@ void page_remove_rmap(struct page *page)
* Leaving it set also helps swapoff to reinstate ptes
* faster for those pages still in swapcache.
*/
out:
if (!anon)
mem_cgroup_end_update_page_stat(page, &locked, &flags);
}
/*