Re: [RFC PATCH RESEND 09/28] mm/mempolicy: mark VMA as locked when changing protection policy

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Sep 6, 2022 at 7:48 AM Laurent Dufour <ldufour@linux.ibm.com> wrote:
>
> Le 01/09/2022 à 19:34, Suren Baghdasaryan a écrit :
> > Protect VMA from concurrent page fault handler while performing VMA
> > protection policy changes.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > ---
> >  mm/mempolicy.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > index b73d3248d976..6be1e5c75556 100644
> > --- a/mm/mempolicy.c
> > +++ b/mm/mempolicy.c
> > @@ -383,8 +383,10 @@ void mpol_rebind_mm(struct mm_struct *mm, nodemask_t *new)
> >       struct vm_area_struct *vma;
> >
> >       mmap_write_lock(mm);
> > -     for (vma = mm->mmap; vma; vma = vma->vm_next)
> > +     for (vma = mm->mmap; vma; vma = vma->vm_next) {
> > +             vma_mark_locked(vma);
> >               mpol_rebind_policy(vma->vm_policy, new);
> > +     }
> >       mmap_write_unlock(mm);
> >  }
> >
> > @@ -632,6 +634,7 @@ unsigned long change_prot_numa(struct vm_area_struct *vma,
> >       struct mmu_gather tlb;
> >       int nr_updated;
> >
> > +     vma_mark_locked(vma);
>
> If I understand that corretly, the VMA itself is not impacted, only the
> PMDs/PTEs, and they are protected using the page table locks.
>
> Am I missing something?

I thought we would not want pages faulting in the VMA for which we are
changing the protection. However I think what you are saying is that
page table locks would already provide a more granular synchronization
with page fault handlers, which makes sense to me. Sounds like we can
skip locking the VMA here as well. Nice!

>
> >       tlb_gather_mmu(&tlb, vma->vm_mm);
> >
> >       nr_updated = change_protection(&tlb, vma, addr, end, PAGE_NONE,
> > @@ -765,6 +768,7 @@ static int vma_replace_policy(struct vm_area_struct *vma,
> >       if (IS_ERR(new))
> >               return PTR_ERR(new);
> >
> > +     vma_mark_locked(vma);
> >       if (vma->vm_ops && vma->vm_ops->set_policy) {
> >               err = vma->vm_ops->set_policy(vma, new);
> >               if (err)
>

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