[PATCH] x86: mm: Fix vmalloc_fault oops during lazy MMU updates

From: Samu Kallio
Date: Wed Feb 20 2013 - 20:45:55 EST

In paravirtualized x86_64 kernels, vmalloc_fault may cause an oops
when lazy MMU updates are enabled, because set_pgd effects are being

One instance of this problem is during process mm cleanup with memory
cgroups enabled. The chain of events is as follows:

- zap_pte_range enables lazy MMU updates
- zap_pte_range eventually calls mem_cgroup_charge_statistics,
which accesses the vmalloc'd mem_cgroup per-cpu stat area
- vmalloc_fault is triggered which tries to sync the corresponding
PGD entry with set_pgd, but the update is deferred
- vmalloc_fault oopses due to a mismatch in the PUD entries

Calling arch_flush_lazy_mmu_mode immediately after set_pgd makes the
changes visible to the consistency checks.

Signed-off-by: Samu Kallio <samu.kallio@xxxxxxxxxxxxxxxxx>
arch/x86/mm/fault.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 027088f..3ba3dba 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -378,10 +378,12 @@ static noinline __kprobes int vmalloc_fault(unsigned long address)
if (pgd_none(*pgd_ref))
return -1;

- if (pgd_none(*pgd))
+ if (pgd_none(*pgd)) {
set_pgd(pgd, *pgd_ref);
- else
+ arch_flush_lazy_mmu_mode();
+ } else {
BUG_ON(pgd_page_vaddr(*pgd) != pgd_page_vaddr(*pgd_ref));
+ }

* Below here mismatches are bugs because these lower tables

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