linux-kernel - Re: [PATCH v2] mm: kmem: make memcg_kmem

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Message-ID: <20200702181726.GA114903@carbon.dhcp.thefacebook.com>
Date:   Thu, 2 Jul 2020 11:17:26 -0700
From:   Roman Gushchin <guro@...com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>, <linux-mm@...ck.org>,
        Vlastimil Babka <vbabka@...e.cz>, <kernel-team@...com>,
        <linux-kernel@...r.kernel.org>,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH v2] mm: kmem: make memcg_kmem_enabled() irreversible

On Thu, Jul 02, 2020 at 11:09:26AM -0700, Roman Gushchin wrote:
> Historically the kernel memory accounting was an opt-in feature, which
> could be enabled for individual cgroups. But now it's not true, and
> it's on by default both on cgroup v1 and cgroup v2.  And as long as a
> user has at least one non-root memory cgroup, the kernel memory
> accounting is on. So in most setups it's either always on (if memory
> cgroups are in use and kmem accounting is not disabled), either always
> off (otherwise).
> 
> memcg_kmem_enabled() is used in many places to guard the kernel memory
> accounting code. If memcg_kmem_enabled() can reverse from returning
> true to returning false (as now), we can't rely on it on release paths
> and have to check if it was on before.
> 
> If we'll make memcg_kmem_enabled() irreversible (always returning true
> after returning it for the first time), it'll make the general logic
> more simple and robust. It also will allow to guard some checks which
> otherwise would stay unguarded.
> 
> Signed-off-by: Roman Gushchin <guro@...com>
> Reported-by: Naresh Kamboju <naresh.kamboju@...aro.org>
> Tested-by: Naresh Kamboju <naresh.kamboju@...aro.org>
> Reviewed-by: Shakeel Butt <shakeelb@...gle.com>
> Acked-by: Vlastimil Babka <vbabka@...e.cz>
> Acked-by: Michal Hocko <mhocko@...e.com>

Andrew,

can you, please, put this patch before my slab series?

Thank you!

Roman