| Message ID | 20180702075049.9157-1-borntraeger@de.ibm.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Mon, 2 Jul 2018 09:50:49 +0200 Christian Borntraeger <borntraeger@de.ibm.com> wrote: > 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. > > ... > > --- 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> > > @@ -1481,7 +1482,7 @@ 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 A reader of this code will wonder why we're checking userfaultfd_armed(). So the writer of this code should add a comment which explains this to them ;) Please.
On 07/02/2018 11:06 PM, Andrew Morton wrote: > On Mon, 2 Jul 2018 09:50:49 +0200 Christian Borntraeger <borntraeger@de.ibm.com> wrote: > >> 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. >> >> ... >> >> --- 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> >> >> @@ -1481,7 +1482,7 @@ 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 > > A reader of this code will wonder why we're checking > userfaultfd_armed(). So the writer of this code should add a comment > which explains this to them ;) Please. > Something like: /* * 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. */ ?
diff --git a/mm/rmap.c b/mm/rmap.c index 6db729dc4c50..e8fa564676b6 100644 --- 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> @@ -1481,7 +1482,7 @@ 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
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. Cc: Martin Schwidefsky <schwidefsky@de.ibm.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Mike Rapoport <rppt@linux.vnet.ibm.com> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com> --- rfc->v2: use userfaultfd_armed mm/rmap.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)