linux-kernel - Re: [PATCH v7 09/19] mm: memcg/slab: charge individual slab objects instead of pages

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Message-ID: <CALvZod4jchG_4gvigzb=iAoi1ytX6HVJe0xCp8b8=cc4NiXENg@mail.gmail.com>
Date:   Mon, 22 Jun 2020 22:19:25 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Roman Gushchin <guro@...com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Linux MM <linux-mm@...ck.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Kernel Team <kernel-team@...com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v7 09/19] mm: memcg/slab: charge individual slab objects
 instead of pages

On Mon, Jun 22, 2020 at 6:58 PM Roman Gushchin <guro@...com> wrote:
>
> Switch to per-object accounting of non-root slab objects.
>
> Charging is performed using obj_cgroup API in the pre_alloc hook.
> Obj_cgroup is charged with the size of the object and the size of
> metadata: as now it's the size of an obj_cgroup pointer.  If the amount of
> memory has been charged successfully, the actual allocation code is
> executed.  Otherwise, -ENOMEM is returned.
>
> In the post_alloc hook if the actual allocation succeeded, corresponding
> vmstats are bumped and the obj_cgroup pointer is saved.  Otherwise, the
> charge is canceled.
>
> On the free path obj_cgroup pointer is obtained and used to uncharge the
> size of the releasing object.
>
> Memcg and lruvec counters are now representing only memory used by active
> slab objects and do not include the free space.  The free space is shared
> and doesn't belong to any specific cgroup.
>
> Global per-node slab vmstats are still modified from
> (un)charge_slab_page() functions.  The idea is to keep all slab pages
> accounted as slab pages on system level.
>
> Signed-off-by: Roman Gushchin <guro@...com>
> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
> ---
[snip]
> +static inline struct kmem_cache *memcg_slab_pre_alloc_hook(struct kmem_cache *s,
> +                                               struct obj_cgroup **objcgp,
> +                                               size_t objects, gfp_t flags)
> +{
> +       struct kmem_cache *cachep;
> +
> +       cachep = memcg_kmem_get_cache(s, objcgp);
> +       if (is_root_cache(cachep))
> +               return s;
> +
> +       if (obj_cgroup_charge(*objcgp, flags, objects * obj_full_size(s))) {
> +               memcg_kmem_put_cache(cachep);

I think you forgot to put obj_cgroup_put(*objcgp) here again.

> +               cachep = NULL;
> +       }
> +
> +       return cachep;
> +}
> +

After the above fix:

Reviewed-by: Shakeel Butt <shakeelb@...gle.com>