On Tue, Feb 28, 2023 at 11:57 PM Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx> 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 <surenb@xxxxxxxxxx>
> > > ---
> > > 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
vma->detached after vma_start_read() helps us avoid handling faults in
the detached VMA.
> >
> > 2) as VMAs that are going to be removed are already locked in vma_prepare(),
> > so I think this hunk could be dropped?
>
> After sending this just realized that I did not consider simple munmap case :)
> But I still think 1) and 3) are valid question.
>
> >
> > > if (mas_store_gfp(mas_detach, vma, GFP_KERNEL))
> > > return -ENOMEM;
> > > diff --git a/mm/nommu.c b/mm/nommu.c
> > > index 57ba243c6a37..2ab162d773e2 100644
> > > --- a/mm/nommu.c
> > > +++ b/mm/nommu.c
> > > @@ -588,6 +588,7 @@ static int delete_vma_from_mm(struct vm_area_struct *vma)
> > > current->pid);
> > > return -ENOMEM;
> > > }
> > > + vma_start_write(vma);
> > > cleanup_vma_from_mm(vma);
> >
> > 3) I think this hunk could be dropped as Per-VMA lock depends on MMU anyway.
Ah, yes, you are right. We can safely remove the changes in nommu.c
Andrew, should I post a fixup or you can make the removal directly in
mm-unstable?
Thanks,
Suren.
> >
> > Thanks,
> > Hyeonggon
> >
> > >
> > > /* remove from the MM's tree and list */
> > > @@ -1519,6 +1520,10 @@ void exit_mmap(struct mm_struct *mm)
> > > */
> > > mmap_write_lock(mm);
> > > for_each_vma(vmi, vma) {
> > > + /*
> > > + * No need to lock VMA because this is the only mm user and no
> > > + * page fault handled can race with it.
> > > + */
> > > cleanup_vma_from_mm(vma);
> > > delete_vma(mm, vma);
> > > cond_resched();
> > > --
> > > 2.39.2.722.g9855ee24e9-goog
> > >
> > >
> >
>
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