[PATCH] mm: memcontrol: raise MEMCG_MAX for charges that fail without reclaiming

[Joe Damato <joe@dama.to>]

Charges that exceed memory.max and return through the nomem label can
raise no event and simply return -ENOMEM.

A non-blocking charge can hit the limit, get rejected, but is not
visible in memory.events.

This was noticed in a production setting where bpf_mem_alloc() attempted
to refill its per-cpu freelists, which triggered a non-blocking charge
while at the limit.

Move the event so that it is raised as soon as the charge is known not
to fit.

Suggested-by: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Joe Damato <joe@dama.to>
---
 mm/memcontrol.c | 15 +++++----------
 1 file changed, 5 insertions(+), 10 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 1271d390b617..3904fe9a7b2e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2683,6 +2683,11 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
 		goto retry;
 	}
 
+	if (!raised_max_event) {
+		__memcg_memory_event(mem_over_limit, MEMCG_MAX, allow_spinning);
+		raised_max_event = true;
+	}
+
 	/*
 	 * Prevent unbounded recursion when reclaim operations need to
 	 * allocate memory. This might exceed the limits temporarily,
@@ -2711,9 +2716,6 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
 	    mm_flags_test(MMF_OOM_SKIP, current->signal->oom_mm))
 		goto nomem;
 
-	__memcg_memory_event(mem_over_limit, MEMCG_MAX, allow_spinning);
-	raised_max_event = true;
-
 	psi_memstall_enter(&pflags);
 	nr_reclaimed = try_to_free_mem_cgroup_pages(mem_over_limit, nr_pages,
 						    gfp_mask, reclaim_options, NULL);
@@ -2773,13 +2775,6 @@ 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, allow_spinning);
-
 	/*
 	 * The allocation either can't fail or will lead to more memory
 	 * being freed very soon.  Allow memory usage go over the limit

base-commit: 3d83758432b5e6ed9507500a57efb0f3af41ee7d
-- 
2.53.0-Meta