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, 4 insertions, 2 deletions

diff --git a/arch/powerpc/kernel/eeh.c b/arch/powerpc/kernel/eeh.c
index a4c62eb0ee48..44b480e3a5af 100644
--- a/arch/powerpc/kernel/eeh.c
+++ b/arch/powerpc/kernel/eeh.c
@@ -334,9 +334,11 @@ static inline unsigned long eeh_token_to_phys(unsigned long token)
         int hugepage_shift;
 
         /*
-         * We won't find hugepages here, iomem
+         * We won't find hugepages here(this is iomem). Hence we are not
+         * worried about _PAGE_SPLITTING/collapse. Also we will not hit
+         * page table free, because of init_mm.
          */
-        ptep = find_linux_pte_or_hugepte(init_mm.pgd, token, &hugepage_shift);
+        ptep = __find_linux_pte_or_hugepte(init_mm.pgd, token, &hugepage_shift);
         if (!ptep)
                 return token;
         WARN_ON(hugepage_shift);