[PATCH v2] 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.

Commit d6e103a757fa ("mm: memcontrol: do not miss MEMCG_MAX events for
enforced allocations") added raised_max_event to cover charges that are
force charged without ever reaching reclaim, but charges that are
rejected outright were left out. Getting an allocation failure without
the corresponding MEMCG_MAX event is unexpected and makes debugging and
monitoring harder.

Raise the event on the way out for rejected charges as well, by routing
the -ENOMEM return through the same exit path that already covers forced
charges. The existing behavior of raising a MEMCG_MAX event on every
charge/reclaim/retry iteration is left unchanged.

Tested with a module that performs accounted GFP_NOWAIT page allocations
from a task in a cgroup at its memory.max, and measures the resulting
memory.events:max delta. Without this patch the rejected charges raise
no event at all; with it the delta matches the number of rejected charges
exactly. A GFP_KERNEL|__GFP_NORETRY control, which reaches reclaim, raises
the same two events per failed charge before and after, confirming the
existing charge/reclaim/retry accounting is unchanged.

Fixes: d6e103a757fa ("mm: memcontrol: do not miss MEMCG_MAX events for enforced allocations")
Cc: stable@vger.kernel.org
Suggested-by: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Joe Damato <joe@dama.to>
---
v2:
  - v1 raised MEMCG_MAX once, as soon as the charge was known not to fit, which
    collapsed the existing per-iteration events raised while a charge loops
    through reclaim and retry. Instead leave that event where it is and route
    the -ENOMEM return through the same exit path that already covers forced
    charges, so only the rejected-charge case changes, as suggested by Shakeel.
  - Add Fixes tag and CC stable, as suggested by Shakeel.

v1: https://lore.kernel.org/cgroups/20260827233119.411152-1-joe@dama.to/

 mm/memcontrol.c | 28 ++++++++++++++++------------
 1 file changed, 16 insertions(+), 12 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 1271d390b617..158b0562e8df 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2656,10 +2656,11 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
 	bool raised_max_event = false;
 	unsigned long pflags;
 	bool allow_spinning = gfpflags_allow_spinning(gfp_mask);
+	int ret = 0;
 
 retry:
 	if (consume_stock(memcg, nr_pages))
-		return 0;
+		return ret;
 
 	if (!allow_spinning)
 		/* Avoid the refill and flush of the older stock */
@@ -2770,16 +2771,11 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
 	 * put the burden of reclaim on regular allocation requests
 	 * and let these go through as privileged allocations.
 	 */
-	if (!(gfp_mask & (__GFP_NOFAIL | __GFP_HIGH)))
-		return -ENOMEM;
+	if (!(gfp_mask & (__GFP_NOFAIL | __GFP_HIGH))) {
+		ret = -ENOMEM;
+		goto out;
+	}
 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
@@ -2789,7 +2785,15 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
 	if (do_memsw_account())
 		page_counter_charge(&memcg->memsw, nr_pages);
 
-	return 0;
+out:
+	/*
+	 * Don't forget to raise a MEMCG_MAX event for forced or rejected
+	 * requests.
+	 */
+	if (!raised_max_event)
+		__memcg_memory_event(mem_over_limit, MEMCG_MAX, allow_spinning);
+
+	return ret;
 
 done_restock:
 	if (batch > nr_pages)
@@ -2848,7 +2852,7 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
 	    !(current->flags & PF_MEMALLOC) &&
 	    gfpflags_allow_blocking(gfp_mask))
 		__mem_cgroup_handle_over_high(gfp_mask);
-	return 0;
+	return ret;
 }
 
 static inline int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
-- 
2.53.0-Meta