[Devel] [PATCH VZ10] mm: fix negative dentry accounting by protecting memcg with rcu_read_lock

[Konstantin Khorenko]

ack

--
Best regards,

Konstantin Khorenko,
Virtuozzo Linux Kernel Team

On 04.08.2025 06:39, Pavel Tikhomirov wrote:
> This fixes the warning:
> 
> WARNING: CPU: 0 PID: 1 at include/linux/memcontrol.h:404 mem_cgroup_from_slab_obj+0x426/0x580
> 
> In this stack:
> 
>    +-> dput
>      +-> __dentry_kill
>        +-> d_lru_del
>          +-> memcg_neg_dentry_dec
>            +-> mem_cgroup_from_slab_obj
>              +-> mem_cgroup_from_obj_folio
>                +-> obj_cgroup_memcg
> 
> Detected when running xfstests-generic/321.
> 
> https://virtuozzo.atlassian.net/browse/VSTOR-112802
> Fixes: 2b4edb1f998a ("mm: per-memcg negative dentries accounting")
> Signed-off-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
> 
> Feature: mm: statistic enhancements
> ---
>   mm/memcontrol.c | 10 ++++++++--
>   1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 59eb819c91052..4512a16d9ede3 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3221,18 +3221,24 @@ unsigned long mem_cgroup_read_nd(struct mem_cgroup *memcg)
>   
>   void memcg_neg_dentry_inc(struct dentry *dentry)
>   {
> -	struct mem_cgroup *memcg = mem_cgroup_from_slab_obj(dentry);
> +	struct mem_cgroup *memcg;
>   
> +	rcu_read_lock();
> +	memcg = mem_cgroup_from_slab_obj(dentry);
>   	if (memcg)
>   		__this_cpu_inc(memcg->vmstats_percpu->nr_dentry_neg);
> +	rcu_read_unlock();
>   }
>   
>   void memcg_neg_dentry_dec(struct dentry *dentry)
>   {
> -	struct mem_cgroup *memcg = mem_cgroup_from_slab_obj(dentry);
> +	struct mem_cgroup *memcg;
>   
> +	rcu_read_lock();
> +	memcg = mem_cgroup_from_slab_obj(dentry);
>   	if (memcg)
>   		__this_cpu_dec(memcg->vmstats_percpu->nr_dentry_neg);
> +	rcu_read_unlock();
>   }
>   
>   /*