linux-kernel - Re: [PATCH v6 09/19] mm: memcg/slab: charge individual slab objects instead of pages

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Date:   Fri, 19 Jun 2020 18:29:22 -0700
From:   Roman Gushchin <guro@...com>
To:     Shakeel Butt <shakeelb@...gle.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 v6 09/19] mm: memcg/slab: charge individual slab objects
 instead of pages

On Fri, Jun 19, 2020 at 05:54:24PM -0700, Shakeel Butt wrote:
> On Mon, Jun 8, 2020 at 4:07 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);
> 
> obj_cgroup_put(objcgp)? Or better to do that in memcg_kmem_put_cache().

Hm, you are right, it's a real issue. It's fixed later in the series by
"mm: memcg/slab: use a single set of kmem_caches for all accounted allocations"
though. Good catch!

I'll re-spin necessary patches to avoid it and ask Andrew to replace them.

Thank you!