From: Christian Borntraeger borntraeger@de.ibm.com Subject: mm: do not drop unused pages when userfaultd is running
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.
Link: http://lkml.kernel.org/r/20180703171854.63981-1-borntraeger@de.ibm.com Signed-off-by: Christian Borntraeger borntraeger@de.ibm.com Cc: Martin Schwidefsky schwidefsky@de.ibm.com Cc: Andrea Arcangeli aarcange@redhat.com Cc: Mike Rapoport rppt@linux.vnet.ibm.com Cc: Janosch Frank frankja@linux.ibm.com Cc: David Hildenbrand david@redhat.com Cc: Cornelia Huck cohuck@redhat.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
mm/rmap.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-)
diff -puN mm/rmap.c~mm-do-not-drop-unused-pages-when-userfaultd-is-running mm/rmap.c --- a/mm/rmap.c~mm-do-not-drop-unused-pages-when-userfaultd-is-running +++ a/mm/rmap.c @@ -64,6 +64,7 @@ #include <linux/backing-dev.h> #include <linux/page_idle.h> #include <linux/memremap.h> +#include <linux/userfaultfd_k.h>
#include <asm/tlbflush.h>
@@ -1481,11 +1482,16 @@ static bool try_to_unmap_one(struct page set_pte_at(mm, address, pvmw.pte, pteval); }
- } else if (pte_unused(pteval)) { + } else if (pte_unused(pteval) && !userfaultfd_armed(vma)) { /* * The guest indicated that the page content is of no * interest anymore. Simply discard the pte, vmscan * will take care of the rest. + * A future reference will then fault in a new zero + * page. When userfaultfd is active, we must not drop + * this page though, as its main user (postcopy + * migration) will not expect userfaults on already + * copied pages. */ dec_mm_counter(mm, mm_counter(page)); /* We have to invalidate as we cleared the pte */ _