Re: linux-next: runtime warning in Linus' tree

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Aug 13, 2020 at 04:46:54PM +1000, Stephen Rothwell wrote:
> [    0.055220][    T0] WARNING: CPU: 0 PID: 0 at mm/memcontrol.c:5220 mem_cgroup_css_alloc+0x350/0x904

> [The line numbers in the final linux next are 5226 and 5141 due to
> later patches.]
> 
> Introduced (or exposed) by commit
> 
>   3e38e0aaca9e ("mm: memcg: charge memcg percpu memory to the parent cgroup")
> 
> This commit actually adds the WARN_ON, so it either adds the bug that
> sets it off, or the bug already existed.
> 
> Unfotunately, the version of this patch in linux-next up tuntil today
> is different.  :-(

Sorry, I made a last-minute request to include these checks in that
patch to make the code a bit more robust, but they trigger a false
positive here. Let's remove them.

---

From de8ea7c96c056c3cbe7b93995029986a158fb9cd Mon Sep 17 00:00:00 2001
From: Johannes Weiner <hannes@cmpxchg.org>
Date: Thu, 13 Aug 2020 10:40:54 -0400
Subject: [PATCH] mm: memcontrol: fix warning when allocating the root cgroup

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@cmpxchg.org>
---
 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