Re: [PATCH] mm/memcontrol: fix warning in mem_cgroup_page_lruvec()

[Vlastimil Babka <vbabka@suse.cz>]

On 1/4/21 6:03 AM, Hugh Dickins wrote:
> Boot a CONFIG_MEMCG=y kernel with "cgroup_disabled=memory" and you are
> met by a series of warnings from the VM_WARN_ON_ONCE_PAGE(!memcg, page)
> recently added to the inline mem_cgroup_page_lruvec().
> 
> An earlier attempt to place that warning, in mem_cgroup_lruvec(), had
> been careful to do so after weeding out the mem_cgroup_disabled() case;
> but was itself invalid because of the mem_cgroup_lruvec(NULL, pgdat) in
> clear_pgdat_congested() and age_active_anon().
> 
> Warning in mem_cgroup_page_lruvec() was once useful in detecting a KSM
> charge bug, so may be worth keeping: but skip if mem_cgroup_disabled().
> 
> Fixes: 9a1ac2288cf1 ("mm/memcontrol:rewrite mem_cgroup_page_lruvec()")
> Signed-off-by: Hugh Dickins <hughd@google.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
> 
>  include/linux/memcontrol.h |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- 5.11-rc2/include/linux/memcontrol.h	2020-12-27 20:39:36.751923135 -0800
> +++ linux/include/linux/memcontrol.h	2021-01-03 19:38:24.822978559 -0800
> @@ -665,7 +665,7 @@ static inline struct lruvec *mem_cgroup_
>  {
>  	struct mem_cgroup *memcg = page_memcg(page);
>  
> -	VM_WARN_ON_ONCE_PAGE(!memcg, page);
> +	VM_WARN_ON_ONCE_PAGE(!memcg && !mem_cgroup_disabled(), page);

Nit: I would reverse the order of conditions as mem_cgroup_disabled() is either
"return true" or a static key. Not that it matters too much on DEBUG_VM configs...

>  	return mem_cgroup_lruvec(memcg, pgdat);
>  }
>  
>