Commit bce73e48 authored by Christian Borntraeger's avatar Christian Borntraeger Committed by Linus Torvalds

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: default avatarChristian Borntraeger <>
Cc: Martin Schwidefsky <>
Cc: Andrea Arcangeli <>
Cc: Mike Rapoport <>
Cc: Janosch Frank <>
Cc: David Hildenbrand <>
Cc: Cornelia Huck <>
Cc: <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent 3951dbf2
......@@ -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 */
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