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

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Message-ID: <CALOAHbDnJyou=MUwPBCEwWeeK+9w2MiOXjpkkcCALExfnsj=_A@mail.gmail.com>
Date: Thu, 16 Oct 2025 16:18:08 +0800
From: Yafang Shao <laoar.shao@...il.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.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 3:21 PM Lorenzo Stoakes
<lorenzo.stoakes@...cle.com> wrote:
>
> 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?

Sure, I will send a v2.

>
> 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

Huh, we've been refactoring a bit too fast and furious since your last
review ;-)

-- 
Regards
Yafang