From 03375203e1da8f4782ec5cd7023eb1b4adfba739 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Michal=20Koutn=C3=BD?= Date: Wed, 25 Feb 2026 19:38:44 +0100 Subject: [PATCH] mm: do not allocate shrinker info with cgroup.memory=nokmem MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit There'd be no work for memcg-aware shrinkers when kernel memory is not accounted per cgroup, so we can skip allocating per memcg shrinker data. This saves some memory, avoids holding shrinker_mutex with O(nr_memcgs) and saves work in shrink_slab_memcg(). Then there are SHRINKER_NONSLAB shrinkers which handle non-kernel memory so nokmem should not disable their per-memcg behavior. Such shrinkers (e.g. deferred_split_shrinker) still need access to per-memcg data (see also commit 0a432dcbeb32e ("mm: shrinker: make shrinker not depend on memcg kmem")). The savings with this patch come on container hosts that create many superblocks (each with own shrinker) but tracking and processing per-memcg data is pointless with nokmem (shrink_slab_memcg() is partially guarded with !memcg_kmem_online already). The patch uses "boottime" predicate mem_cgroup_kmem_disabled() (not memcg_kmem_online()) to avoid mistakenly un-MEMCG_AWARE-ing shrinkers registered before first non-root memcg is mkdir'd. [mkoutny@suse.com: update comment, per Qi Zheng] Link: https://lkml.kernel.org/r/20260309-cgroup-ml-nokmem-shrinker-v2-1-3e7a7eefb6c9@suse.com Link: https://lkml.kernel.org/r/20260225-cgroup-ml-nokmem-shrinker-v1-1-d703899bdda4@suse.com Signed-off-by: Michal Koutný Reviewed-by: Roman Gushchin Acked-by: Qi Zheng Reviewed-by: Muchun Song Cc: Dave Chinner Cc: Jan Kara Signed-off-by: Andrew Morton --- mm/shrinker.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/mm/shrinker.c b/mm/shrinker.c index 0f90d63afdeb..c23086bccf4d 100644 --- a/mm/shrinker.c +++ b/mm/shrinker.c @@ -219,6 +219,8 @@ static int shrinker_memcg_alloc(struct shrinker *shrinker) if (mem_cgroup_disabled()) return -ENOSYS; + if (mem_cgroup_kmem_disabled() && !(shrinker->flags & SHRINKER_NONSLAB)) + return -ENOSYS; mutex_lock(&shrinker_mutex); id = idr_alloc(&shrinker_idr, shrinker, 0, 0, GFP_KERNEL); @@ -721,6 +723,7 @@ struct shrinker *shrinker_alloc(unsigned int flags, const char *fmt, ...) * - non-memcg-aware shrinkers * - !CONFIG_MEMCG * - memcg is disabled by kernel command line + * - non-slab shrinkers: when memcg kmem is disabled */ size = sizeof(*shrinker->nr_deferred); if (flags & SHRINKER_NUMA_AWARE)