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/[email protected]
Signed-off-by: Roman Gushchin <[email protected]>
Reported-by: Yafang Shao <[email protected]>
Acked-by: Shakeel Butt <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Muchun Song <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/memcontrol.c

@@ -2577,6 +2577,7 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
 	bool passed_oom = false;
 	bool may_swap = true;
 	bool drained = false;
+	bool raised_max_event = false;
 	unsigned long pflags;
 
 retry:
@@ -2616,6 +2617,7 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
 		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,
@@ -2682,6 +2684,13 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
 	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