Re: [PATCH v2 3/3] userfaultfd: use per-vma locks in userfaultfd operations

[Lokesh Gidra <lokeshgidra@google.com>]

On Mon, Feb 5, 2024 at 1:47 PM Suren Baghdasaryan <surenb@google.com> wrote:
>
> On Wed, Jan 31, 2024 at 1:41 PM Liam R. Howlett <Liam.Howlett@oracle.com> wrote:
> >
> > * Lokesh Gidra <lokeshgidra@google.com> [240130 21:49]:
> > > On Mon, Jan 29, 2024 at 6:58 PM Liam R. Howlett <Liam.Howlett@oracle.com> wrote:
> > > >
> > > > * Lokesh Gidra <lokeshgidra@google.com> [240129 19:28]:
> > > > > On Mon, Jan 29, 2024 at 12:53 PM Suren Baghdasaryan <surenb@google.com> wrote:
> > > > > >
> > > >
> >
> > ...
> >
> > > >
> > > > > Your suggestion is definitely simpler and easier to follow, but due to
> > > > > the overflow situation that Suren pointed out, I would still need to
> > > > > keep the locking/boolean dance, no? IIUC, even if I were to return
> > > > > EAGAIN to the userspace, there is no guarantee that subsequent ioctls
> > > > > on the same vma will succeed due to the same overflow, until someone
> > > > > acquires and releases mmap_lock in write-mode.
> > > > > Also, sometimes it seems insufficient whether we managed to lock vma
> > > > > or not. For instance, lock_vma_under_rcu() checks if anon_vma (for
> > > > > anonymous vma) exists. If not then it bails out.
> > > > > So it seems to me that we have to provide some fall back in
> > > > > userfaultfd operations which executes with mmap_lock in read-mode.
> > > >
> > > > Fair enough, what if we didn't use the sequence number and just locked
> > > > the vma directly?
> > >
> > > Looks good to me, unless someone else has any objections.
> > > >
> > > > /* This will wait on the vma lock, so once we return it's locked */
> > > > void vma_aquire_read_lock(struct vm_area_struct *vma)
> > > > {
> > > >         mmap_assert_locked(vma->vm_mm);
> > > >         down_read(&vma->vm_lock->lock);
> > > > }
> > > >
> > > > struct vm_area_struct *lock_vma(struct mm_struct *mm,
> > > >         unsigned long addr))    /* or some better name.. */
> > > > {
> > > >         struct vm_area_struct *vma;
> > > >
> > > >         vma = lock_vma_under_rcu(mm, addr);
> > > >         if (vma)
> > > >                 return vma;
> > > >
> > > >         mmap_read_lock(mm);
> > > >         /* mm sequence cannot change, no mm writers anyways.
> > > >          * find_mergeable_anon_vma is only a concern in the page fault
> > > >          * path
> > > >          * start/end won't change under the mmap_lock
> > > >          * vma won't become detached as we have the mmap_lock in read
> > > >          * We are now sure no writes will change the VMA
> > > >          * So let's make sure no other context is isolating the vma
> > > >          */
> > > >         vma = lookup_vma(mm, addr);
> > > >         if (vma)
> > > We can take care of anon_vma as well here right? I can take a bool
> > > parameter ('prepare_anon' or something) and then:
> > >
> > >            if (vma) {
> > >                     if (prepare_anon && vma_is_anonymous(vma)) &&
> > > !anon_vma_prepare(vma)) {
> > >                                       vma = ERR_PTR(-ENOMEM);
> > >                                       goto out_unlock;
> > >                    }
> > > >                 vma_aquire_read_lock(vma);
> > >            }
> > > out_unlock:
> > > >         mmap_read_unlock(mm);
> > > >         return vma;
> > > > }
> >
> > Do you need this?  I didn't think this was happening in the code as
> > written?  If you need it I would suggest making it happen always and
> > ditch the flag until a user needs this variant, but document what's
> > going on in here or even have a better name.
>
> I think yes, you do need this. I can see calls to anon_vma_prepare()
> under mmap_read_lock() protection in both mfill_atomic_hugetlb() and
> in mfill_atomic(). This means, just like in the pagefault path, we
> modify vma->anon_vma under mmap_read_lock protection which guarantees
> that adjacent VMAs won't change. This is important because
> __anon_vma_prepare() uses find_mergeable_anon_vma() that needs the
> neighboring VMAs to be stable. Per-VMA lock guarantees stability of
> the VMA we locked but not of its neighbors, therefore holding per-VMA
> lock while calling anon_vma_prepare() is not enough. The solution
> Lokesh suggests would call anon_vma_prepare() under mmap_read_lock and
> therefore would avoid the issue.
>
Thanks, Suren.
anon_vma_prepare() is also called in validate_move_areas() via move_pages().
>
> >
> > Thanks,
> > Liam