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 Signed-off-by: Joe Damato --- 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