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Subject[PATCH 7/9] x86: skip check for spurious faults for non-present faults
If a fault on a kernel address is due to a non-present page, then it
cannot be the result of stale TLB entry from a protection change (RO
to RW or NX to X). Thus the pagetable walk in spurious_fault() can be

This avoids spurious_fault() oopsing in some cases if the pagetables
it attempts to walk are not accessible. This obscures the location of
the original fault.

This also fixes a crash with Xen PV guests when they access entries in
the M2P corresponding to device MMIO regions. The M2P is mapped
(read-only) by Xen into the kernel address space of the guest and this
mapping may contains holes for non-RAM regions. Read faults will
result in calls to spurious_fault(), but because the page tables for
the M2P mappings are not accessible by the guest the pagetable walk
would fault.

This was not normally a problem as MMIO mappings would not normally
result in a M2P lookup because of the use of the _PAGE_IOMAP bit the
PTE. However, removing the _PAGE_IOMAP bit requires M2P lookups for
MMIO mappings as well.

Signed-off-by: David Vrabel <>
Reported-by: Konrad Rzeszutek Wilk <>
Tested-by: Konrad Rzeszutek Wilk <>
x86 maintainers, this is a prerequisite for removing Xen's usage of
_PAGE_IOMAP so I think this is best merged via the Xen tree.
arch/x86/mm/fault.c | 6 ++++--
1 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 8e57229..c39e249 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -936,8 +936,10 @@ spurious_fault(unsigned long error_code, unsigned long address)
pte_t *pte;
int ret;

- /* Reserved-bit violation or user access to kernel space? */
- if (error_code & (PF_USER | PF_RSVD))
+ /* Only check for spurious faults on supervisor write or
+ instruction faults. */
+ if (error_code != (PF_WRITE | PF_PROT)
+ && error_code != (PF_INSTR | PF_PROT))
return 0;

pgd = init_mm.pgd + pgd_index(address);

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