Subject: Re: [PATCH v4 18/33] mm: write-lock VMAs before removing them from VMA tree

From: Hyeonggon Yoo <[email-masked]>
Date: Thu, 2 Mar 2023 00:53:45 +0000

On Wed, Mar 01, 2023 at 10:42:48AM -0800, Suren Baghdasaryan wrote:
> On Wed, Mar 1, 2023 at 10:34 AM Suren Baghdasaryan <[email-masked]> wrote:
> >
> > On Tue, Feb 28, 2023 at 11:57 PM Hyeonggon Yoo <[email-masked]> wrote:
> > >
> > > On Wed, Mar 01, 2023 at 07:43:33AM +0000, Hyeonggon Yoo wrote:
> > > > On Mon, Feb 27, 2023 at 09:36:17AM -0800, Suren Baghdasaryan wrote:
> > > > > Write-locking VMAs before isolating them ensures that page fault
> > > > > handlers don't operate on isolated VMAs.
> > > > >
> > > > > Signed-off-by: Suren Baghdasaryan <[email-masked]>
> > > > > ---
> > > > >  mm/mmap.c  | 1 +
> > > > >  mm/nommu.c | 5 +++++
> > > > >  2 files changed, 6 insertions(+)
> > > > >
> > > > > diff --git a/mm/mmap.c b/mm/mmap.c
> > > > > index 1f42b9a52b9b..f7ed357056c4 100644
> > > > > --- a/mm/mmap.c
> > > > > +++ b/mm/mmap.c
> > > > > @@ -2255,6 +2255,7 @@ int split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
> > > > >  static inline int munmap_sidetree(struct vm_area_struct *vma,
> > > > >                                struct ma_state *mas_detach)
> > > > >  {
> > > > > +   vma_start_write(vma);
> > > > >     mas_set_range(mas_detach, vma->vm_start, vma->vm_end - 1);
> > > >
> > > > I may be missing something, but have few questions:
> > > >
> > > >       1) Why does a writer need to both write-lock a VMA and mark the VMA detached
> > > >          when unmapping it, isn't it enough to just only write-lock a VMA?
> >
> > We need to mark the VMA detached to avoid handling page fault in a
> > detached VMA. The possible scenario is:
> >
> > lock_vma_under_rcu
> >   vma = mas_walk(&mas)
> >                                                         munmap_sidetree
> >                                                           vma_start_write(vma)
> >
> > mas_store_gfp() // remove VMA from the tree
> >                                                           vma_end_write_all()
> >   vma_start_read(vma)
> >   // we locked the VMA but it is not part of the tree anymore.
> >
> > So, marking the VMA locked before vma_end_write_all() and checking
> 
> Sorry, I should have said "marking the VMA *detached* before
> vma_end_write_all() and checking vma->detached after vma_start_read()
> helps us avoid handling faults in the detached VMA."
> 
> > vma->detached after vma_start_read() helps us avoid handling faults in
> > the detached VMA.

Thank you for explanation, that makes sense!

By the way, if there are no 32bit users of Per-VMA lock (are there?),
"detached" bool could be a VMA flag (i.e. making it depend on 64BIT
and selecting ARCH_USES_HIGH_VMA_FLAGS)

Thanks,
Hyeonggon