linux-kernel - Re: [PATCH v1] mm/mempolicy: fix migrate_to_node() assuming there is at least one VMA in a MM

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Message-ID: <m4p5ngz7l4hgavwysczmliqrgumlx6dxg35jjwlpcmqtzrpmsk@q2wwxruumhrl>
Date: Fri, 22 Nov 2024 09:44:12 -0500
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org,
        syzbot+3511625422f7aa637f0d@...kaller.appspotmail.com,
        stable@...r.kernel.org, Christoph Lameter <cl@...ux.com>
Subject: Re: [PATCH v1] mm/mempolicy: fix migrate_to_node() assuming there is
 at least one VMA in a MM

* David Hildenbrand <david@...hat.com> [241122 04:32]:
> On 22.11.24 07:19, Andrew Morton wrote:
> > On Wed, 20 Nov 2024 15:27:46 -0500 "Liam R. Howlett" <Liam.Howlett@...cle.com> wrote:
> > 
> > > I hate the extra check because syzbot can cause this as this should
> > > basically never happen in real life, but it seems we have to add it.
> > 
> > So..
> > 
> > --- a/mm/mempolicy.c~mm-mempolicy-fix-migrate_to_node-assuming-there-is-at-least-one-vma-in-a-mm-fix
> > +++ a/mm/mempolicy.c
> > @@ -1080,7 +1080,7 @@ static long migrate_to_node(struct mm_st
> >   	mmap_read_lock(mm);
> >   	vma = find_vma(mm, 0);
> > -	if (!vma) {
> > +	if (unlikely(!vma)) {
> >   		mmap_read_unlock(mm);
> >   		return 0;
> >   	}
> > _
> > 
> > ?
> 
> Why not, at least for documentation purposes. Because I don't think this is
> any fast-path we really care about, so expect the runtime effects to be
> mostly negligible. Thanks!

The next email we get about this will be a bot with a micro benchmark
performance drop.

Really though, I'm happy either way because of what David said.

Thanks,
Liam