doc: cgroup: update note about conditions when oom killer is invoked

Starting from v4.19 commit 29ef680 ("memcg, oom: move out_of_memory
back to the charge path") cgroup oom killer is no longer invoked only from
page faults.  Now it implements the same semantics as global OOM killer:
allocation context invokes OOM killer and keeps retrying until success.

Link: http://lkml.kernel.org/r/158894738928.208854.5244393925922074518.stgit@buzz
Signed-off-by: Konstantin Khlebnikov <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Randy Dunlap <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Stephen Rothwell <[email protected]>

Documentation/admin-guide/cgroup-v2.rst

@@ -1170,6 +1170,13 @@ PAGE_SIZE multiple when read back.
 	Under certain circumstances, the usage may go over the limit
 	temporarily.
 
+	In default configuration regular 0-order allocation always
+	succeed unless OOM killer choose current task as a victim.
+
+	Some kinds of allocations don't invoke the OOM killer.
+	Caller could retry them differently, return into userspace
+	as -ENOMEM or silently ignore in cases like disk readahead.
+
 	This is the ultimate protection mechanism.  As long as the
 	high limit is used and monitored properly, this limit's
 	utility is limited to providing the final safety net.
@@ -1226,17 +1233,9 @@ PAGE_SIZE multiple when read back.
 		The number of time the cgroup's memory usage was
 		reached the limit and allocation was about to fail.
 
-		Depending on context result could be invocation of OOM
-		killer and retrying allocation or failing allocation.
-
-		Failed allocation in its turn could be returned into
-		userspace as -ENOMEM or silently ignored in cases like
-		disk readahead.  For now OOM in memory cgroup kills
-		tasks iff shortage has happened inside page fault.
-
 		This event is not raised if the OOM killer is not
 		considered as an option, e.g. for failed high-order
-		allocations.
+		allocations or if caller asked to not retry attempts.
 
 	  oom_kill
 		The number of processes belonging to this cgroup