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

Starting from v4.19 commit 29ef680a ("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.

[akpm@linux-foundation.org: fixes per Randy]
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Roman Gushchin <guro@fb.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Link: http://lkml.kernel.org/r/158894738928.208854.5244393925922074518.stgit@buzzSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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 allocations always
+	succeed unless OOM killer chooses 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