[PATCH] mm: memcontrol: fix warning when allocating the root cgroup

[Johannes Weiner]

Commit 3e38e0aaca9e ("mm: memcg: charge memcg percpu memory to the
parent cgroup") adds memory tracking to the memcg kernel structures
themselves to make cgroups liable for the memory they are consuming
through the allocation of child groups (which can be significant).

This code is a bit awkward as it's spread out through several
functions: The outermost function does memalloc_use_memcg(parent) to
set up current->active_memcg, which designates which cgroup to charge,
and the inner functions pass GFP_ACCOUNT to request charging for
specific allocations. To make sure this dependency is satisfied at all
times - to make sure we don't randomly charge whoever is calling the
functions - the inner functions warn on !current->active_memcg.

However, this triggers a false warning when the root memcg itself is
allocated. No parent exists in this case, and so current->active_memcg
is rightfully NULL. It's a false positive, not indicative of a bug.

Delete the warnings for now, we can revisit this later.

Fixes: 3e38e0aaca9e ("mm: memcg: charge memcg percpu memory to the parent cgroup")
Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
 mm/memcontrol.c | 6 ------
 1 file changed, 6 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index d59fd9af6e63..9d87082e64aa 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5137,9 +5137,6 @@ static int alloc_mem_cgroup_per_node_info(struct mem_cgroup *memcg, int node)
 	if (!pn)
 		return 1;
 
-	/* We charge the parent cgroup, never the current task */
-	WARN_ON_ONCE(!current->active_memcg);
-
 	pn->lruvec_stat_local = alloc_percpu_gfp(struct lruvec_stat,
 						 GFP_KERNEL_ACCOUNT);
 	if (!pn->lruvec_stat_local) {
@@ -5222,9 +5219,6 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
 		goto fail;
 	}
 
-	/* We charge the parent cgroup, never the current task */
-	WARN_ON_ONCE(!current->active_memcg);
-
 	memcg->vmstats_local = alloc_percpu_gfp(struct memcg_vmstats_percpu,
 						GFP_KERNEL_ACCOUNT);
 	if (!memcg->vmstats_local)
-- 
2.28.0