Commit bce73e48 authored Jul 13, 2018 by Christian Borntraeger Committed by Linus Torvalds Jul 14, 2018

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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 3951dbf2

mm/rmap.c

+7 −1

Original line number	Diff line number	Diff line
		@@ -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 page, struct vm_area_struct vma,
		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 */