memcg, fsnotify: no oom-kill for remote memcg charging

[ Upstream commit ec165450968b26298bd1c373de37b0ab6d826b33 ]

Commit d46eb14b735b ("fs: fsnotify: account fsnotify metadata to
kmemcg") added remote memcg charging for fanotify and inotify event
objects.  The aim was to charge the memory to the listener who is
interested in the events but without triggering the OOM killer.
Otherwise there would be security concerns for the listener.

At the time, oom-kill trigger was not in the charging path.  A parallel
work added the oom-kill back to charging path i.e.  commit 29ef680ae7c2
("memcg, oom: move out_of_memory back to the charge path").  So to not
trigger oom-killer in the remote memcg, explicitly add
__GFP_RETRY_MAYFAIL to the fanotigy and inotify event allocations.

Link: http://lkml.kernel.org/r/20190514212259.156585-2-shakeelb@google.com
Signed-off-by: Shakeel Butt <shakeelb@google.com>
Reviewed-by: Roman Gushchin <guro@fb.com>
Acked-by: Jan Kara <jack@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

fs/notify/fanotify/fanotify.c

@@ -148,10 +148,13 @@ struct fanotify_event_info *fanotify_alloc_event(struct fsnotify_group *group,
 	/*
 	 * For queues with unlimited length lost events are not expected and
 	 * can possibly have security implications. Avoid losing events when
-	 * memory is short.
+	 * memory is short. For the limited size queues, avoid OOM killer in the
+	 * target monitoring memcg as it may have security repercussion.
 	 */
 	if (group->max_events == UINT_MAX)
 		gfp |= __GFP_NOFAIL;
+	else
+		gfp |= __GFP_RETRY_MAYFAIL;
 
 	/* Whoever is interested in the event, pays for the allocation. */
 	memalloc_use_memcg(group->memcg);

fs/notify/inotify/inotify_fsnotify.c

@@ -99,9 +99,13 @@ int inotify_handle_event(struct fsnotify_group *group,
 	i_mark = container_of(inode_mark, struct inotify_inode_mark,
 			      fsn_mark);
 
-	/* Whoever is interested in the event, pays for the allocation. */
+	/*
+	 * Whoever is interested in the event, pays for the allocation. Do not
+	 * trigger OOM killer in the target monitoring memcg as it may have
+	 * security repercussion.
+	 */
 	memalloc_use_memcg(group->memcg);
-	event = kmalloc(alloc_len, GFP_KERNEL_ACCOUNT);
+	event = kmalloc(alloc_len, GFP_KERNEL_ACCOUNT | __GFP_RETRY_MAYFAIL);
 	memalloc_unuse_memcg();
 
 	if (unlikely(!event)) {