memcg: document cgroup dirty memory interfaces

Document cgroup dirty memory interfaces and statistics.

[[e-mail]: fix use_hierarchy description]
Signed-off-by: Andrea Righi <[e-mail]>
Signed-off-by: Greg Thelen <[e-mail]>
Cc: KAMEZAWA Hiroyuki <[e-mail]>
Cc: Daisuke Nishimura <[e-mail]>
Cc: Balbir Singh <[e-mail]>
Cc: Minchan Kim <[e-mail]>
Cc: Wu Fengguang <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/cgroups/memory.txt

···
 pgpgin		- # of pages paged in (equivalent to # of charging events).
 pgpgout		- # of pages paged out (equivalent to # of uncharging events).
 swap		- # of bytes of swap usage
+dirty		- # of bytes that are waiting to get written back to the disk.
+writeback	- # of bytes that are actively being written back to the disk.
+nfs_unstable	- # of bytes sent to the NFS server, but not yet committed to
+		the actual storage.
 inactive_anon	- # of bytes of anonymous memory and swap cache memory on
 		LRU list.
 active_anon	- # of bytes of anonymous and swap cache memory on active
···
 total_pgpgin		- sum of all children's "pgpgin"
 total_pgpgout		- sum of all children's "pgpgout"
 total_swap		- sum of all children's "swap"
+total_dirty		- sum of all children's "dirty"
+total_writeback		- sum of all children's "writeback"
+total_nfs_unstable	- sum of all children's "nfs_unstable"
 total_inactive_anon	- sum of all children's "inactive_anon"
 total_active_anon	- sum of all children's "active_anon"
 total_inactive_file	- sum of all children's "inactive_file"
···
 
 You can reset failcnt by writing 0 to failcnt file.
 # echo 0 > .../memory.failcnt
+
+5.5 dirty memory
+
+Control the maximum amount of dirty pages a cgroup can have at any given time.
+
+Limiting dirty memory is like fixing the max amount of dirty (hard to reclaim)
+page cache used by a 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 interface is equivalent to the procfs interface: /proc/sys/vm/dirty_*.  It
+is possible to configure a limit to trigger both a direct writeback or a
+background writeback performed by per-bdi flusher threads.  The root cgroup
+memory.dirty_* control files are read-only and match the contents of
+the /proc/sys/vm/dirty_* files.
+
+Per-cgroup dirty limits can be set using the following files in the cgroupfs:
+
+- memory.dirty_ratio: the amount of dirty memory (expressed as a percentage of
+  cgroup memory) at which a process generating dirty pages will itself start
+  writing out dirty data.
+
+- memory.dirty_limit_in_bytes: the amount of dirty memory (expressed in bytes)
+  in the cgroup at which a process generating dirty pages will start itself
+  writing out dirty data.  Suffix (k, K, m, M, g, or G) can be used to indicate
+  that value is kilo, mega or gigabytes.
+
+  Note: memory.dirty_limit_in_bytes is the counterpart of memory.dirty_ratio.
+  Only one of them may be specified at a time.  When one is written it is
+  immediately taken into account to evaluate the dirty memory limits and the
+  other appears as 0 when read.
+
+- memory.dirty_background_ratio: the amount of dirty memory of the cgroup
+  (expressed as a percentage of cgroup memory) at which background writeback
+  kernel threads will start writing out dirty data.
+
+- memory.dirty_background_limit_in_bytes: the amount of dirty memory (expressed
+  in bytes) in the cgroup at which background writeback kernel threads will
+  start writing out dirty data.  Suffix (k, K, m, M, g, or G) can be used to
+  indicate that value is kilo, mega or gigabytes.
+
+  Note: memory.dirty_background_limit_in_bytes is the counterpart of
+  memory.dirty_background_ratio.  Only one of them may be specified at a time.
+  When one is written it is immediately taken into account to evaluate the dirty
+  memory limits and the other appears as 0 when read.
+
+A cgroup may contain more dirty memory than its dirty limit.  This is possible
+because of the principle that the first cgroup to touch a page is charged for
+it.  Subsequent page counting events (dirty, writeback, nfs_unstable) are also
+counted to the originally charged cgroup.
+
+Example: If page is allocated by a cgroup A task, then the page is charged to
+cgroup A.  If the page is later dirtied by a task in cgroup B, then the cgroup A
+dirty count will be incremented.  If cgroup A is over its dirty limit but cgroup
+B is not, then dirtying a cgroup A page from a cgroup B task may push cgroup A
+over its dirty limit without throttling the dirtying cgroup B task.
+
+When use_hierarchy=0, each cgroup has dirty memory usage and limits.
+System-wide dirty limits are also consulted.  Dirty memory consumption is
+checked against both system-wide and per-cgroup dirty limits.
+
+The current implementation does not enforce per-cgroup dirty limits when
+use_hierarchy=1.  System-wide dirty limits are used for processes in such
+cgroups.  Attempts to read memory.dirty_* files return the system-wide
+values.  Writes to the memory.dirty_* files return error.  An enhanced
+implementation is needed to check the chain of parents to ensure that no
+dirty limit is exceeded.
 
 6. Hierarchy support