On Wed, 21 Mar 2012 18:52:04 +0900
KAMEZAWA Hiroyuki [off-list ref] wrote:

As discussed before, I post this to fix the spec and implementation of task moving.
Then, do you think what target kernel version should be ? 3.4/3.5 ?
but yes, it may be late for 3.4....

Well, the key information here is "what effect does the bug have upon
users".

In documentation, it's said that 'shared anon are not moved'.
But in implementation, the check was wrong.

  if (!move_anon() || page_mapcount(page) > 2)

Ah, memcg has been moving shared anon pages for a long time.

Then, here is a discussion about handling of shared anon pages.

 - It's complex
 - Now, shared file caches are moved in force.
 - It adds unclear check as page_mapcount(). To do correct check,
   we should check swap users, etc.
 - No one notice this implementation behavior. So, no one get benefit
   from the design.
 - In general, once task is moved to a cgroup for running, it will not
   be moved....
 - Finally, we have control knob as memory.move_charge_at_immigrate.

Here is a patch to allow moving shared pages, completely. This makes
memcg simpler and fix current broken code.

Note:
 IIUC, libcgroup's cgroup daemon moves tasks after exec().
 So, it's not affected.
 libcgroup's command "cgexec" does move itsef to a memcg and call exec()
 without fork(). it's not affected.

Changelog:
 - fixed PageAnon() check.
 - remove call of lookup_swap_cache()
 - fixed Documentation.

But you forgot to tell us :(

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