[RFC PATCH RESEND 10/28] mm/mmap: mark VMAs as locked in vma_adjust

[Suren Baghdasaryan]

On Tue, Sep 6, 2022 at 8:35 AM Laurent Dufour <ldufour at linux.ibm.com> wrote:
>
> Le 01/09/2022 à 19:34, Suren Baghdasaryan a écrit :
> > vma_adjust modifies a VMA and possibly its neighbors. Mark them as locked
> > before making the modifications.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb at google.com>
> > ---
> >  mm/mmap.c | 11 ++++++++++-
> >  1 file changed, 10 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index f89c9b058105..ed58cf0689b2 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -710,6 +710,10 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
> >       long adjust_next = 0;
> >       int remove_next = 0;
> >
> > +     vma_mark_locked(vma);
> > +     if (next)
> > +             vma_mark_locked(next);
> > +
>
> I was wondering if the VMAs insert and expand should be locked too.
>
> For expand, I can't see any valid reason, but for insert, I'm puzzled.
> I would think that it is better to lock the VMA to be inserted but I can't
> really justify that.
>
> It may be nice to detail why this is not need to lock insert and expand here.

'expand' is always locked before it's passed to __vma_adjust() by
vma_merge(). It has to be locked before we decide "Can it merge with
the predecessor?" here
https://elixir.bootlin.com/linux/latest/source/mm/mmap.c#L1201 because
a change in VMA can affect that decision. I spent many hours tracking
the issue caused by not locking the VMA before making this decision.
It might be good to add a comment about this...

AFAIKT 'insert' is only used by __split_vma() and it's always a brand
new VMA which is not yet linked into mm->mmap. Any reason
__vma_adjust() should lock it?

>
> >       if (next && !insert) {
> >               struct vm_area_struct *exporter = NULL, *importer = NULL;
> >
> > @@ -754,8 +758,11 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
> >                        * If next doesn't have anon_vma, import from vma after
> >                        * next, if the vma overlaps with it.
> >                        */
> > -                     if (remove_next == 2 && !next->anon_vma)
> > +                     if (remove_next == 2 && !next->anon_vma) {
> >                               exporter = next->vm_next;
> > +                             if (exporter)
> > +                                     vma_mark_locked(exporter);
> > +                     }
> >
> >               } else if (end > next->vm_start) {
> >                       /*
> > @@ -931,6 +938,8 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
> >                        * "vma->vm_next" gap must be updated.
> >                        */
> >                       next = vma->vm_next;
> > +                     if (next)
> > +                             vma_mark_locked(next);
> >               } else {
> >                       /*
> >                        * For the scope of the comment "next" and
>
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