On Wed, Mar 01, 2023 at 10:42:48AM -0800, Suren Baghdasaryan wrote:
> On Wed, Mar 1, 2023 at 10:34 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> >
> > 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
>
> 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
