Re: [PATCH/RFC] mm: do not drop unused pages when userfaultd is running

[Mike Rapoport]

On Fri, Jun 29, 2018 at 08:51:23AM +0200, Christian Borntraeger wrote:
> 
> 
> On 06/28/2018 02:39 PM, Christian Borntraeger wrote:
> > KVM guests on s390 can notify the host of unused pages. This can result
> > in pte_unused callbacks to be true for KVM guest memory.
> > 
> > If a page is unused (checked with pte_unused) we might drop this page
> > instead of paging it. This can have side-effects on userfaultd, when the
> > page in question was already migrated:
> > 
> > The next access of that page will trigger a fault and a user fault
> > instead of faulting in a new and empty zero page. As QEMU does not
> > expect a userfault on an already migrated page this migration will fail.
> > 
> > The most straightforward solution is to ignore the pte_unused hint if a
> > userfault context is active for this VMA.
> > 
> > Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
> > Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> > ---
> >  mm/rmap.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index 6db729dc4c50..3f3a72aa99f2 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1481,7 +1481,7 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
> >  				set_pte_at(mm, address, pvmw.pte, pteval);
> >  			}
> >  
> > -		} else if (pte_unused(pteval)) {
> > +		} else if (pte_unused(pteval) && !vma->vm_userfaultfd_ctx.ctx) {
> 
> FWIW, this needs a fix for !CONFIG_USERFAULTFD.

There's userfaultfd_armed() in include/linux/userfaultfd_k.h. Just
s/!vma->vm_userfaultfd_ctx.ctx/!userfaultfd_armed(vma)

> Still: more opinions on the patch itself? 

If the only use case for pte_unused() hint is guest notification for host,
the patch seems Ok to me.

> >  			/*
> >  			 * The guest indicated that the page content is of no
> >  			 * interest anymore. Simply discard the pte, vmscan
> > 
> 

-- 
Sincerely yours,
Mike.