From: Andrea Righi on
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_ratio / memory.dirty_bytes
and memory.dirty_background_ratio / memory.dirty_background_bytes in
- start to write-out (directly or background) when the cgroup limits are

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 /proc/sys/vm/dirty_ratio|dirty_bytes and

Changelog (v4 -> v5)
* fixed a potential deadlock between lock_page_cgroup and mapping->tree_lock
(I'm not sure I did the right thing for this point, so review and tests are
very welcome)
* introduce inc/dec functions to update file cache accounting
* export only a restricted subset of mem_cgroup_stat_index flags
* fixed a bug in determine_dirtyable_memory() to correctly return the local
memcg dirtyable memory
* always use global dirty memory settings in calc_period_shift()

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