From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mike Rapoport Subject: Re: [PATCH/RFC] mm: do not drop unused pages when userfaultd is running Date: Fri, 29 Jun 2018 23:46:05 +0300 Message-ID: <20180629204604.GF4799@rapoport-lnx> References: <20180628123916.96106-1-borntraeger@de.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-mm@kvack.org, linux-s390@vger.kernel.org, kvm@vger.kernel.org, Janosch Frank , David Hildenbrand , Cornelia Huck , linux-kernel@vger.kernel.org, Martin Schwidefsky , Andrea Arcangeli To: Christian Borntraeger Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org 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 > > Cc: Andrea Arcangeli > > Cc: stable@vger.kernel.org > > Signed-off-by: Christian Borntraeger > > --- > > 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.