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

Suren Baghdasaryan
2025-01-13T19:11:35Z

On Mon, Jan 13, 2025 at 9:13 AM Lorenzo Stoakes
<lorenzo.stoakes@oracle.com> wrote:
>
> 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@oracle.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()?

Sure. I'll change vma_assert_attached()/vma_assert_detached() to use
WARN_ON_ONCE() and to return a bool (see also my reply in the patch
[0/17]).

>
> >
> > >
> > > >
> > > > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > > > Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
> > >
> > > But regardless, prima facie, this looks fine, so:
> > >
> > > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.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
> > > >