On Fri, Feb 17, 2012 at 06:27:43PM +0900, KAMEZAWA Hiroyuki wrote:

quoted

From 5bf592d432e4552db74e94a575afcd83aa652a9d Mon Sep 17 00:00:00 2001

From: KAMEZAWA Hiroyuki <redacted>
Date: Thu, 2 Feb 2012 11:49:59 +0900
Subject: [PATCH 4/6] memcg: use new logic for page stat accounting.

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 [off-list ref].
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 introduce 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.

Changes since v4
 - removed unused argument *lock
 - added more comments.
Changes since v3
 - fixed typos

Signed-off-by: KAMEZAWA Hiroyuki <redacted>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

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