[RFC] [PATCH 0/2] memcg: per cgroup dirty limit

[Andrea Righi]

Control the maximum amount of dirty pages a cgroup can have at any given time.

Per cgroup dirty limit is like fixing the max amount of dirty (hard to reclaim)
page cache used by any cgroup. So, in case of multiple cgroup writers, they
will not be able to consume more than their designated share of dirty pages and
will be forced to perform write-out if they cross that limit.

The overall design is the following:

 - account dirty pages per cgroup
 - limit the number of dirty pages via memory.dirty_bytes in cgroupfs
 - start to write-out in balance_dirty_pages() when the cgroup or global limit
   is exceeded

This feature is supposed to be strictly connected to any underlying IO
controller implementation, so we can stop increasing dirty pages in VM layer
and enforce a write-out before any cgroup will consume the global amount of
dirty pages defined by the /proc/sys/vm/dirty_ratio|dirty_bytes limit.

TODO:
 - handle the migration of tasks across different cgroups (a page may be set
   dirty when a task runs in a cgroup and cleared after the task is moved to
   another cgroup).
 - provide an appropriate documentation (in Documentation/cgroups/memory.txt)

-Andrea
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