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

[Harry Yoo <harry.yoo@oracle.com>]

On Fri, May 09, 2025 at 08:11:55PM -0700, Shakeel Butt wrote:
> Hi Andrew,
> 
> On Fri, May 09, 2025 at 06:26:32PM -0700, Andrew Morton wrote:
> > On Fri,  9 May 2025 16:28:55 -0700 Shakeel Butt <shakeel.butt@linux.dev> wrote:
> > 
> > > BPF programs can trigger memcg charged kernel allocations in nmi
> > > context. However memcg charging infra for kernel memory is not equipped
> > > to handle nmi context. This series adds support for kernel memory
> > > charging for nmi context.
> > 
> > The patchset adds quite a bit of material to core MM on behalf of a
> > single caller.  So can we please take a close look at why BPF is doing
> > this?
> > 
> > What would be involved in changing BPF to avoid doing this, or of
> > changing BPF to handle things locally?  What would be the end-user
> > impact of such an alteration?  IOW, what is the value to our users of
> > the present BPF behavior?
> > 
> 
> 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.

If kmalloc gains NMI-context support, preallocation would no longer be
necessary, eliminating its memory overhead which has been observed to
reach up to 1.5GB in Meta's fleet [1].

[1] https://lore.kernel.org/linux-mm/20250327145159.99799-1-alexei.starovoitov@gmail.com

-- 
Cheers,
Harry / Hyeonggon