linux-kernel - Re: [PATCH 0/4] memcg: nmi-safe kmem charging

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Message-ID: <45bf0c55-42c4-4af7-8e77-ac8dba2768dd@suse.cz>
Date: Mon, 12 May 2025 16:52:41 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Shakeel Butt <shakeel.butt@...ux.dev>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>,
 Roman Gushchin <roman.gushchin@...ux.dev>,
 Muchun Song <muchun.song@...ux.dev>, Alexei Starovoitov <ast@...nel.org>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>, bpf@...r.kernel.org,
 linux-mm@...ck.org, cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
 Meta kernel team <kernel-team@...a.com>
Subject: Re: [PATCH 0/4] memcg: nmi-safe kmem charging

On 5/10/25 05:11, Shakeel Butt wrote:
> Before answering the questions, let me clarify that this series is
> continuation of the work which added similar support for page allocator
> and related memcg charging and now the work is happening for
> kmalloc/slab allocations. Alexei has a proposal on reentrant kmalloc and
> here I am providing how memcg charging for that (reentrant kmalloc)
> should work.
> 
> Next let me take a stab in answering the questions and BPF folks can
> correct me if I am wrong. From what I understand, users can attach BPF
> programs at almost any place in kernel and those BPF programs can do
> memory allocations. This line of work is to make those allocations work
> if the any such BPF attach point is triggered in mni context.
> 
> Before this line of work (reentrant page and slab allocators), I think
> BPF had its internal cache but it was very limited and can easily fail
> allocation requests (please BPF folks correct me if I am wrong). This
> was discussed in LSFMM this year as well.
> 
> Now regarding the impact to the users. First there will not be any
> negative impact on the non-users of this feature. For the value this
> feature will provide to users, I think this line of work will make BPF
> programs of the users more reliable with better allocation behavior.
> BPF folks, please add more comments for the value of these features.

Yes and I think this part of cover letter is also important:

> There will be a followup series which will make kernel memory charging
> reentrant for irq and will be able to do without disabling irqs.

The "without disabling irqs" part will improve performance for all users.