powerpc/mm/thp: Make page table walk safe against thp split/collapse

We can disable a THP split or a hugepage collapse by disabling irq.
We do send IPI to all the cpus in the early part of split/collapse,
and disabling local irq ensure we don't make progress with
split/collapse. If the THP is getting split we return NULL from
find_linux_pte_or_hugepte(). For all the current callers it should be ok.
We need to be careful if we want to use returned pte_t pointer outside
the irq disabled region. W.r.t to THP split, the pfn remains the same,
but then a hugepage collapse will result in a pfn change. There are
few steps we can take to avoid a hugepage collapse.One way is to take page
reference inside the irq disable region. Other option is to take
mmap_sem so that a parallel collapse will not happen. We can also
disable collapse by taking pmd_lock. Another method used by kvm
subsystem is to check whether we had a mmu_notifer update in between
using mmu_notifier_retry().

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

1 files changed, 10 insertions, 1 deletions

diff --git a/arch/powerpc/include/asm/pgtable.h b/arch/powerpc/include/asm/pgtable.h
index 92fe01c..11a3863 100644
--- a/arch/powerpc/include/asm/pgtable.h
+++ b/arch/powerpc/include/asm/pgtable.h
@@ -247,8 +247,17 @@ extern int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
 #define pmd_large(pmd)		0
 #define has_transparent_hugepage() 0
 #endif
-pte_t *find_linux_pte_or_hugepte(pgd_t *pgdir, unsigned long ea,
+pte_t *__find_linux_pte_or_hugepte(pgd_t *pgdir, unsigned long ea,
 				 unsigned *shift);
+static inline pte_t *find_linux_pte_or_hugepte(pgd_t *pgdir, unsigned long ea,
+					       unsigned *shift)
+{
+	if (!arch_irqs_disabled()) {
+		pr_info("%s called with irq enabled\n", __func__);
+		dump_stack();
+	}
+	return __find_linux_pte_or_hugepte(pgdir, ea, shift);
+}
 #endif /* __ASSEMBLY__ */
 
 #endif /* __KERNEL__ */