mm: memcontrol: do not miss MEMCG_MAX events for enforced allocations

Yafang Shao reported an issue related to the accounting of bpf memory:
if a bpf map is charged indirectly for memory consumed from an
interrupt context and allocations are enforced, MEMCG_MAX events are
not raised.

It's not/less of an issue in a generic case because consequent
allocations from a process context will trigger the direct reclaim and
MEMCG_MAX events will be raised. However a bpf map can belong to a
dying/abandoned memory cgroup, so there will be no allocations from a
process context and no MEMCG_MAX events will be triggered.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Roman Gushchin <[e-mail]>
Reported-by: Yafang Shao <[e-mail]>
Acked-by: Shakeel Butt <[e-mail]>
Acked-by: Michal Hocko <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Muchun Song <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

mm/memcontrol.c

···
 	bool passed_oom = false;
 	bool may_swap = true;
 	bool drained = false;
+	bool raised_max_event = false;
 	unsigned long pflags;
 
 retry:
···
 		goto nomem;
 
 	memcg_memory_event(mem_over_limit, MEMCG_MAX);
+	raised_max_event = true;
 
 	psi_memstall_enter(&pflags);
 	nr_reclaimed = try_to_free_mem_cgroup_pages(mem_over_limit, nr_pages,
···
 	if (!(gfp_mask & (__GFP_NOFAIL | __GFP_HIGH)))
 		return -ENOMEM;
 force:
+	/*
+	 * If the allocation has to be enforced, don't forget to raise
+	 * a MEMCG_MAX event.
+	 */
+	if (!raised_max_event)
+		memcg_memory_event(mem_over_limit, MEMCG_MAX);
+
 	/*
 	 * The allocation either can't fail or will lead to more memory
 	 * being freed very soon.  Allow memory usage go over the limit