Re: [PATCH v3 7/7] mm: fix /proc/kpagecgroup interface for slab pages

[Shakeel Butt]

From: Roman Gushchin <guro@xxxxxx>
Date: Wed, May 8, 2019 at 1:40 PM
To: Andrew Morton, Shakeel Butt
Cc: <linux-mm@xxxxxxxxx>, <linux-kernel@xxxxxxxxxxxxxxx>,
<kernel-team@xxxxxx>, Johannes Weiner, Michal Hocko, Rik van Riel,
Christoph Lameter, Vladimir Davydov, <cgroups@xxxxxxxxxxxxxxx>, Roman
Gushchin

> Switching to an indirect scheme of getting mem_cgroup pointer for
> !root slab pages broke /proc/kpagecgroup interface for them.
>
> Let's fix it by learning page_cgroup_ino() how to get memcg
> pointer for slab pages.
>
> Reported-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> Signed-off-by: Roman Gushchin <guro@xxxxxx>
> ---
>  mm/memcontrol.c  |  5 ++++-
>  mm/slab.h        | 21 +++++++++++++++++++++
>  mm/slab_common.c |  1 +
>  3 files changed, 26 insertions(+), 1 deletion(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 6e4d9ed16069..8114838759f6 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -494,7 +494,10 @@ ino_t page_cgroup_ino(struct page *page)
>         unsigned long ino = 0;
>
>         rcu_read_lock();
> -       memcg = READ_ONCE(page->mem_cgroup);
> +       if (PageSlab(page))
> +               memcg = memcg_from_slab_page(page);
> +       else
> +               memcg = READ_ONCE(page->mem_cgroup);
>         while (memcg && !(memcg->css.flags & CSS_ONLINE))
>                 memcg = parent_mem_cgroup(memcg);
>         if (memcg)
> diff --git a/mm/slab.h b/mm/slab.h
> index acdc1810639d..cb684fbe2cc2 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -256,6 +256,22 @@ static inline struct kmem_cache *memcg_root_cache(struct kmem_cache *s)
>         return s->memcg_params.root_cache;
>  }
>

Can you please document the preconditions of this function? It seems
like it must be PageSlab() then why need to check PageTail and do
compound_head().


> +static inline struct mem_cgroup *memcg_from_slab_page(struct page *page)
> +{
> +       struct kmem_cache *s;
> +
> +       WARN_ON_ONCE(!rcu_read_lock_held());
> +
> +       if (PageTail(page))
> +               page = compound_head(page);
> +
> +       s = READ_ONCE(page->slab_cache);
> +       if (s && !is_root_cache(s))
> +               return rcu_dereference(s->memcg_params.memcg);
> +
> +       return NULL;
> +}
> +
>  static __always_inline int memcg_charge_slab(struct page *page,
>                                              gfp_t gfp, int order,
>                                              struct kmem_cache *s)
> @@ -338,6 +354,11 @@ static inline struct kmem_cache *memcg_root_cache(struct kmem_cache *s)
>         return s;
>  }
>
> +static inline struct mem_cgroup *memcg_from_slab_page(struct page *page)
> +{
> +       return NULL;
> +}
> +
>  static inline int memcg_charge_slab(struct page *page, gfp_t gfp, int order,
>                                     struct kmem_cache *s)
>  {
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 36673a43ed31..0cfdad0a0aac 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -253,6 +253,7 @@ static void memcg_unlink_cache(struct kmem_cache *s)
>                 list_del(&s->memcg_params.kmem_caches_node);
>                 mem_cgroup_put(rcu_dereference_protected(s->memcg_params.memcg,
>                         lockdep_is_held(&slab_mutex)));
> +               rcu_assign_pointer(s->memcg_params.memcg, NULL);
>         }
>  }
>  #else
> --
> 2.20.1
>