linux-kernel - Re: [PATCH v2 4/5] mm: kmem: scoped objcg protection

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Message-ID: <20231012134726.kx3rcm7vkgxe3nib@google.com>
Date:   Thu, 12 Oct 2023 13:47:26 +0000
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Roman Gushchin <roman.gushchin@...ux.dev>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Muchun Song <muchun.song@...ux.dev>,
        Dennis Zhou <dennis@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Naresh Kamboju <naresh.kamboju@...aro.org>
Subject: Re: [PATCH v2 4/5] mm: kmem: scoped objcg protection

On Mon, Oct 09, 2023 at 05:09:28PM -0700, Roman Gushchin wrote:
> Switch to a scope-based protection of the objcg pointer on slab/kmem
> allocation paths. Instead of using the get_() semantics in the
> pre-allocation hook and put the reference afterwards, let's rely
> on the fact that objcg is pinned by the scope.
> 
> It's possible because:
> 1) if the objcg is received from the current task struct, the task is
>    keeping a reference to the objcg.
> 2) if the objcg is received from an active memcg (remote charging),
>    the memcg is pinned by the scope and has a reference to the
>    corresponding objcg.
> 
> Signed-off-by: Roman Gushchin (Cruise) <roman.gushchin@...ux.dev>

Other than one nit below:

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

> ---
>  include/linux/memcontrol.h |  6 +++++
>  mm/memcontrol.c            | 47 ++++++++++++++++++++++++++++++++++++--
>  mm/slab.h                  | 10 +++-----
>  3 files changed, 54 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 277690af383d..0e53b890f063 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -1769,6 +1769,12 @@ bool mem_cgroup_kmem_disabled(void);
>  int __memcg_kmem_charge_page(struct page *page, gfp_t gfp, int order);
>  void __memcg_kmem_uncharge_page(struct page *page, int order);
>  
> +/*
> + * The returned objcg pointer is safe to use without additional
> + * protection within a scope, refer to the implementation for the
> + * additional details.
> + */

Let's add the definition of scope in the above comment as well like
set_active_memcg and slab memcg hooks.