linux-kernel - Re: [PATCH v9 05/17] mm: mark vmas detached upon exit

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Message-ID: <15720844-b970-4b37-b475-bf6e7c72113e@lucifer.local>
Date: Mon, 13 Jan 2025 17:13:42 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: akpm@...ux-foundation.org, peterz@...radead.org, willy@...radead.org,
        liam.howlett@...cle.com, david.laight.linux@...il.com, mhocko@...e.com,
        vbabka@...e.cz, hannes@...xchg.org, mjguzik@...il.com,
        oliver.sang@...el.com, mgorman@...hsingularity.net, david@...hat.com,
        peterx@...hat.com, oleg@...hat.com, dave@...olabs.net,
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        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v9 05/17] mm: mark vmas detached upon exit

On Mon, Jan 13, 2025 at 09:02:50AM -0800, Suren Baghdasaryan wrote:
> On Mon, Jan 13, 2025 at 4:05 AM Lorenzo Stoakes
> <lorenzo.stoakes@...cle.com> wrote:
> >
> > On Fri, Jan 10, 2025 at 08:25:52PM -0800, Suren Baghdasaryan wrote:
> > > When exit_mmap() removes vmas belonging to an exiting task, it does not
> > > mark them as detached since they can't be reached by other tasks and they
> > > will be freed shortly. Once we introduce vma reuse, all vmas will have to
> > > be in detached state before they are freed to ensure vma when reused is
> > > in a consistent state. Add missing vma_mark_detached() before freeing the
> > > vma.
> >
> > Hmm this really makes me worry that we'll see bugs from this detached
> > stuff, do we make this assumption anywhere else I wonder?
>
> This is the only place which does not currently detach the vma before
> freeing it. If someone tries adding a case like that in the future,
> they will be met with vma_assert_detached() inside vm_area_free().

OK good to know!

Again, I wonder if we should make these assertions stronger as commented
elsewhere, because if we see them in production isn't that worth an actual
non-debug WARN_ON_ONCE()?

>
> >
> > >
> > > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> > > Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
> >
> > But regardless, prima facie, this looks fine, so:
> >
> > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> >
> > > ---
> > >  mm/vma.c | 6 ++++--
> > >  1 file changed, 4 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/mm/vma.c b/mm/vma.c
> > > index b9cf552e120c..93ff42ac2002 100644
> > > --- a/mm/vma.c
> > > +++ b/mm/vma.c
> > > @@ -413,10 +413,12 @@ void remove_vma(struct vm_area_struct *vma, bool unreachable)
> > >       if (vma->vm_file)
> > >               fput(vma->vm_file);
> > >       mpol_put(vma_policy(vma));
> > > -     if (unreachable)
> > > +     if (unreachable) {
> > > +             vma_mark_detached(vma);
> > >               __vm_area_free(vma);
> > > -     else
> > > +     } else {
> > >               vm_area_free(vma);
> > > +     }
> > >  }
> > >
> > >  /*
> > > --
> > > 2.47.1.613.gc27f4b7a9f-goog
> > >