Re: [PATCH] mm: Don't fault around userfaultfd-registered regions on reads

[Peter Xu <peterx@redhat.com>]

On Fri, Nov 27, 2020 at 10:16:05AM +0200, Mike Rapoport wrote:
> On Thu, Nov 26, 2020 at 05:23:59PM -0500, Peter Xu wrote:
> > Faulting around for reads are in most cases helpful for the performance so that
> > continuous memory accesses may avoid another trip of page fault.  However it
> > may not always work as expected.
> > 
> > For example, userfaultfd registered regions may not be the best candidate for
> > pre-faults around the reads.
> > 
> > For missing mode uffds, fault around does not help because if the page cache
> > existed, then the page should be there already.  If the page cache is not
> > there, nothing else we can do, either.  If the fault-around code is destined to
> > be helpless for userfault-missing vmas, then ideally we can skip it.
> > 
> > For wr-protected mode uffds, errornously fault in those pages around could lead
> > to threads accessing the pages without uffd server's awareness.  For example,
> > when punching holes on uffd-wp registered shmem regions, we'll first try to
> > unmap all the pages before evicting the page cache but without locking the
> > page (please refer to shmem_fallocate(), where unmap_mapping_range() is called
> > before shmem_truncate_range()).  When fault-around happens near a hole being
> > punched, we might errornously fault in the "holes" right before it will be
> > punched.  Then there's a small window before the page cache was finally
> > dropped, and after the page will be writable again (NOTE: the uffd-wp protect
> > information is totally lost due to the pre-unmap in shmem_fallocate(), so the
> > page can be writable within the small window).  That's severe data loss.
> > 
> > Let's grant the userspace full control of the uffd-registered ranges, rather
> > than trying to do the tricks.
> > 
> > Cc: Hugh Dickins <hughd@google.com>
> > Cc: Andrea Arcangeli <aarcange@redhat.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> 
> One nit below, except that
> 
> Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>

Thanks!

> > +static inline bool vma_registered_userfaultfd(struct vm_area_struct *vma)
> > +{
> > +	return userfaultfd_missing(vma) || userfaultfd_wp(vma);
> > +}
> 
> We have userfaultfd_armed() that does exectly this, don't we?

Yes, will fix that up.

-- 
Peter Xu