Re: [PATCH v1 1/2] mm/userfaultfd: don't place zeropages when zeropages are disallowed

[Peter Xu]

On Thu, Mar 21, 2024 at 11:29:45PM +0100, David Hildenbrand wrote:
> On 21.03.24 23:20, Peter Xu wrote:
> > On Thu, Mar 21, 2024 at 10:59:53PM +0100, David Hildenbrand wrote:
> > > s390x must disable shared zeropages for processes running VMs, because
> > > the VMs could end up making use of "storage keys" or protected
> > > virtualization, which are incompatible with shared zeropages.
> > > 
> > > Yet, with userfaultfd it is possible to insert shared zeropages into
> > > such processes. Let's fallback to simply allocating a fresh zeroed
> > > anonymous folio and insert that instead.
> > > 
> > > mm_forbids_zeropage() was introduced in commit 593befa6ab74 ("mm: introduce
> > > mm_forbids_zeropage function"), briefly before userfaultfd went
> > > upstream.
> > > 
> > > Note that we don't want to fail the UFFDIO_ZEROPAGE request like we do
> > > for hugetlb, it would be rather unexpected. Further, we also
> > > cannot really indicated "not supported" to user space ahead of time: it
> > > could be that the MM disallows zeropages after userfaultfd was already
> > > registered.
> > > 
> > > Fixes: c1a4de99fada ("userfaultfd: mcopy_atomic|mfill_zeropage: UFFDIO_COPY|UFFDIO_ZEROPAGE preparation")
> > > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> > 
> > Reviewed-by: Peter Xu <peterx@xxxxxxxxxx>
> > 
> > Still, a few comments below.
> > 
> > > ---
> > >   mm/userfaultfd.c | 35 +++++++++++++++++++++++++++++++++++
> > >   1 file changed, 35 insertions(+)
> > > 
> > > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > > index 712160cd41eca..1d1061ccd1dea 100644
> > > --- a/mm/userfaultfd.c
> > > +++ b/mm/userfaultfd.c
> > > @@ -316,6 +316,38 @@ static int mfill_atomic_pte_copy(pmd_t *dst_pmd,
> > >   	goto out;
> > >   }
> > > +static int mfill_atomic_pte_zeroed_folio(pmd_t *dst_pmd,
> > > +		 struct vm_area_struct *dst_vma, unsigned long dst_addr)
> > > +{
> > > +	struct folio *folio;
> > > +	int ret;
> > 
> > nitpick: we can set -ENOMEM here, then
> > 
> > > +
> > > +	folio = vma_alloc_zeroed_movable_folio(dst_vma, dst_addr);
> > > +	if (!folio)
> > > +		return -ENOMEM;
> > 
> > return ret;
> > 
> > > +
> > > +	ret = -ENOMEM;
> > 
> > drop.
> 
> Sure!
> 
> > 
> > > +	if (mem_cgroup_charge(folio, dst_vma->vm_mm, GFP_KERNEL))
> > > +		goto out_put;
> > > +
> > > +	/*
> > > +	 * The memory barrier inside __folio_mark_uptodate makes sure that
> > > +	 * preceding stores to the page contents become visible before
> > > +	 * the set_pte_at() write.
> > > +	 */
> > 
> > This comment doesn't apply.  We can drop it.
> > 
> 
> I thought the same until I spotted that comment (where uffd originally
> copied this from I strongly assume) in do_anonymous_page().
> 
> "Preceding stores" here are: zeroing out the memory.

Ah.. that's okay then.

Considering that userfault used to be pretty cautious on such ordering, as
its specialty to involve many user updates on the page, would you mind we
mention those details out?

	/*
	 * __folio_mark_uptodate contains the memory barrier to make sure 
         * the page updates to the zero page will be visible before
	 * installing the pgtable entries.  See do_anonymous_page().
	 */

Or anything better than my wordings.

Thanks!

-- 
Peter Xu