The patch titled
Subject: mm: do not drop unused pages when userfaultd is running
has been added to the -mm tree. Its filename is
mm-do-not-drop-unused-pages-when-userfaultd-is-running.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-do-not-drop-unused-pages-when-userfaultd-is-running.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-do-not-drop-unused-pages-when-userfaultd-is-running.patch
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------------------------------------------------------
From: Christian Borntraeger <borntraeger@xxxxxxxxxx>
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@xxxxxxxxxx
Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx>
Cc: Janosch Frank <frankja@xxxxxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Cornelia Huck <cohuck@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
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 */
_
Patches currently in -mm which might be from borntraeger@xxxxxxxxxx are
mm-do-not-drop-unused-pages-when-userfaultd-is-running.patch
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