linux-kernel - [PATCH 4.14 33/54] mm: do not drop unused pages when userfaultd is running

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Message-Id: <20180716073457.056365615@linuxfoundation.org>
Date:   Mon, 16 Jul 2018 09:35:30 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 4.14 33/54] mm: do not drop unused pages when userfaultd is running

4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Christian Borntraeger <borntraeger@...ibm.com>

commit bce73e4842390f7b7309c8e253e139db71288ac3 upstream.

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@...ibm.com>
Cc: Martin Schwidefsky <schwidefsky@...ibm.com>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: Mike Rapoport <rppt@...ux.vnet.ibm.com>
Cc: Janosch Frank <frankja@...ux.ibm.com>
Cc: David Hildenbrand <david@...hat.com>
Cc: Cornelia Huck <cohuck@...hat.com>
Cc: <stable@...r.kernel.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 mm/rmap.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

--- a/mm/rmap.c
+++ b/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>
 
@@ -1476,11 +1477,16 @@ 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));
 		} else if (IS_ENABLED(CONFIG_MIGRATION) &&