Re: [PATCH v6 06/16] mm: allow vma_start_read_locked/vma_start_read_locked_nested to fail

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Dec 17, 2024 at 3:31 AM Lokesh Gidra <lokeshgidra@google.com> wrote:
>
> On Mon, Dec 16, 2024 at 11:24 AM Suren Baghdasaryan <surenb@google.com> wrote:
> >
> > With upcoming replacement of vm_lock with vm_refcnt, we need to handle a
> > possibility of vma_start_read_locked/vma_start_read_locked_nested failing
> > due to refcount overflow. Prepare for such possibility by changing these
> > APIs and adjusting their users.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > Cc: Lokesh Gidra <lokeshgidra@google.com>
> > ---
> >  include/linux/mm.h |  6 ++++--
> >  mm/userfaultfd.c   | 17 ++++++++++++-----
> >  2 files changed, 16 insertions(+), 7 deletions(-)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 689f5a1e2181..0ecd321c50b7 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -747,10 +747,11 @@ static inline bool vma_start_read(struct vm_area_struct *vma)
> >   * not be used in such cases because it might fail due to mm_lock_seq overflow.
> >   * This functionality is used to obtain vma read lock and drop the mmap read lock.
> >   */
> > -static inline void vma_start_read_locked_nested(struct vm_area_struct *vma, int subclass)
> > +static inline bool vma_start_read_locked_nested(struct vm_area_struct *vma, int subclass)
> >  {
> >         mmap_assert_locked(vma->vm_mm);
> >         down_read_nested(&vma->vm_lock.lock, subclass);
> > +       return true;
> >  }
> >
> >  /*
> > @@ -759,10 +760,11 @@ static inline void vma_start_read_locked_nested(struct vm_area_struct *vma, int
> >   * not be used in such cases because it might fail due to mm_lock_seq overflow.
> >   * This functionality is used to obtain vma read lock and drop the mmap read lock.
> >   */
> > -static inline void vma_start_read_locked(struct vm_area_struct *vma)
> > +static inline bool vma_start_read_locked(struct vm_area_struct *vma)
> >  {
> >         mmap_assert_locked(vma->vm_mm);
> >         down_read(&vma->vm_lock.lock);
> > +       return true;
> >  }
> >
> >  static inline void vma_end_read(struct vm_area_struct *vma)
> > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > index bc9a66ec6a6e..79e8ae676f75 100644
> > --- a/mm/userfaultfd.c
> > +++ b/mm/userfaultfd.c
> > @@ -85,7 +85,8 @@ static struct vm_area_struct *uffd_lock_vma(struct mm_struct *mm,
> >         mmap_read_lock(mm);
> >         vma = find_vma_and_prepare_anon(mm, address);
> >         if (!IS_ERR(vma))
> > -               vma_start_read_locked(vma);
> > +               if (!vma_start_read_locked(vma))
> > +                       vma = ERR_PTR(-EAGAIN);
> >
> >         mmap_read_unlock(mm);
> >         return vma;
> > @@ -1483,10 +1484,16 @@ static int uffd_move_lock(struct mm_struct *mm,
> >         mmap_read_lock(mm);
> >         err = find_vmas_mm_locked(mm, dst_start, src_start, dst_vmap, src_vmap);
> >         if (!err) {
> > -               vma_start_read_locked(*dst_vmap);
> > -               if (*dst_vmap != *src_vmap)
> > -                       vma_start_read_locked_nested(*src_vmap,
> > -                                               SINGLE_DEPTH_NESTING);
> > +               if (!vma_start_read_locked(*dst_vmap)) {
>
> I think you mistakenly reversed the condition. This block should be
> executed if we manage to lock dst_vma successfully.

Oops. Sorry, you are right. That above condition should have been
reversed. I'll fix it in the next revision.
Thanks!

> > +                       if (*dst_vmap != *src_vmap) {
> > +                               if (!vma_start_read_locked_nested(*src_vmap,
> > +                                                       SINGLE_DEPTH_NESTING)) {
> > +                                       vma_end_read(*dst_vmap);
> > +                                       err = -EAGAIN;
> > +                               }
> > +                       }
> > +               } else
> > +                       err = -EAGAIN;
> >         }
> >         mmap_read_unlock(mm);
> >         return err;
> > --
> > 2.47.1.613.gc27f4b7a9f-goog
> >