• KAMEZAWA Hiroyuki's avatar
    memcg: fix/change behavior of shared anon at moving task · 4b91355e
    KAMEZAWA Hiroyuki authored
    This patch changes memcg's behavior at task_move().
    
    At task_move(), the kernel scans a task's page table and move the changes
    for mapped pages from source cgroup to target cgroup.  There has been a
    bug at handling shared anonymous pages for a long time.
    
    Before patch:
      - The spec says 'shared anonymous pages are not moved.'
      - The implementation was 'shared anonymoys pages may be moved'.
        If page_mapcount <=2, shared anonymous pages's charge were moved.
    
    After patch:
      - The spec says 'all anonymous pages are moved'.
      - The implementation is 'all anonymous pages are moved'.
    
    Considering usage of memcg, this will not affect user's experience.
    'shared anonymous' pages only exists between a tree of processes which
    don't do exec().  Moving one of process without exec() seems not sane.
    For example, libcgroup will not be affected by this change.  (Anyway, no
    one noticed the implementation for a long time...)
    
    Below is a discussion log:
    
     - current spec/implementation are complex
     - Now, shared file caches are moved
     - 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.
    Suggested-by: default avatarHugh Dickins <hughd@google.com>
    Signed-off-by: default avatarKAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    Acked-by: default avatarMichal Hocko <mhocko@suse.cz>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
    Cc: Glauber Costa <glommer@parallels.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    4b91355e
memory.txt 29 KB