linux-kernel - Re: [RFC PATCH v10 mm-new 6/9] bpf: mark mm->owner as __safe_rcu_or

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Message-ID: <ebf60722-34e1-4607-b6a7-595fc2091998@lucifer.local>
Date: Thu, 16 Oct 2025 08:21:22 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Yafang Shao <laoar.shao@...il.com>
Cc: Andrii Nakryiko <andrii.nakryiko@...il.com>, akpm@...ux-foundation.org,
        david@...hat.com, ziy@...dia.com, baolin.wang@...ux.alibaba.com,
        Liam.Howlett@...cle.com, npache@...hat.com, ryan.roberts@....com,
        dev.jain@....com, hannes@...xchg.org, usamaarif642@...il.com,
        gutierrez.asier@...wei-partners.com, willy@...radead.org,
        ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        ameryhung@...il.com, rientjes@...gle.com, corbet@....net,
        21cnbao@...il.com, shakeel.butt@...ux.dev, tj@...nel.org,
        lance.yang@...ux.dev, rdunlap@...radead.org, bpf@...r.kernel.org,
        linux-mm@...ck.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v10 mm-new 6/9] bpf: mark mm->owner as
 __safe_rcu_or_null

On Thu, Oct 16, 2025 at 02:42:43PM +0800, Yafang Shao wrote:
> On Thu, Oct 16, 2025 at 12:36 AM Andrii Nakryiko
> <andrii.nakryiko@...il.com> wrote:
> >
> > On Wed, Oct 15, 2025 at 7:18 AM Yafang Shao <laoar.shao@...il.com> wrote:
> > >
> > > When CONFIG_MEMCG is enabled, we can access mm->owner under RCU. The
> > > owner can be NULL. With this change, BPF helpers can safely access
> > > mm->owner to retrieve the associated task from the mm. We can then make
> > > policy decision based on the task attribute.
> > >
> > > The typical use case is as follows,
> > >
> > >   bpf_rcu_read_lock(); // rcu lock must be held for rcu trusted field
> > >   @owner = @mm->owner; // mm_struct::owner is rcu trusted or null
> > >   if (!@...er)
> > >       goto out;
> > >
> > >   /* Do something based on the task attribute */
> > >
> > > out:
> > >   bpf_rcu_read_unlock();
> > >
> > > Suggested-by: Andrii Nakryiko <andrii@...nel.org>
> > > Signed-off-by: Yafang Shao <laoar.shao@...il.com>
> > > Acked-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> > > ---
> > >  kernel/bpf/verifier.c | 3 +++
> > >  1 file changed, 3 insertions(+)
> > >
> >
> > I thought you were going to send this and next patches outside of your
> > thp patch set to land them sooner, as they don't have dependency on
> > the rest of the patches and are useful on their own?
>
> Thanks for your reminder.
> They have been sent separately:
>
>   https://lore.kernel.org/bpf/20251016063929.13830-1-laoar.shao@gmail.com/

Could we respin this as a v2 without them then please so we can sensibly keep
the two separate?

Thanks!

>
> --
> Regards
> Yafang

I do intend to have a look through the various conversations on this, just
catching up after 2 weeks vacation :) in the kernel this is an eternity, even
during the merge window it seems :P

Cheers, Lorenzo