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Date:   Sat, 9 Jul 2022 02:03:55 +0000
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Subject: Re: [PATCH bpf-next] bpf: reparent bpf maps on memcg offlining
From:   Shakeel Butt <shakeelb@google.com>
To:     Roman Gushchin <roman.gushchin@linux.dev>
Cc:     bpf@vger.kernel.org, Alexei Starovoitov <ast@kernel.org>,
        linux-kernel@vger.kernel.org
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On Fri, Jul 08, 2022 at 05:28:58PM -0700, Roman Gushchin wrote:
> The memory consumed by a mpf map is always accounted to the memory
> cgroup of the process which created the map. The map can outlive
> the memory cgroup if it's used by processes in other cgroups or
> is pinned on bpffs. In this case the map pins the original cgroup
> in the dying state.
> 
> For other types of objects (slab objects, non-slab kernel allocations,
> percpu objects and recently LRU pages) there is a reparenting process
> implemented: on cgroup offlining charged objects are getting
> reassigned to the parent cgroup. Because all charges and statistics
> are fully recursive it's a fairly cheap operation.
> 
> For efficiency and consistency with other types of objects, let's do
> the same for bpf maps. Fortunately thanks to the objcg API, the
> required changes are minimal.
> 
> Please, note that individual allocations (slabs, percpu and large
> kmallocs) already have the reparenting mechanism. This commit adds
> it to the saved map->memcg pointer by replacing it to map->objcg.
> Because dying cgroups are not visible for a user and all charges are
> recursive, this commit doesn't bring any behavior changes for a user.
> 
> Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>

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