[PATCHv3 12/41] thp: handle write-protection faults for file THP

[Kirill A. Shutemov]

For filesystems that wants to be write-notified (has mkwrite), we will
encount write-protection faults for huge PMDs in shared mappings.

The easiest way to handle them is to clear the PMD and let it refault as
wriable.

Signed-off-by: Kirill A. Shutemov <kirill.shute...@linux.intel.com>
---
 mm/memory.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index 83be99d9d8a1..aad8d5c6311f 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3451,8 +3451,17 @@ static int wp_huge_pmd(struct fault_env *fe, pmd_t 
orig_pmd)
                return fe->vma->vm_ops->pmd_fault(fe->vma, fe->address, fe->pmd,
                                fe->flags);
 
+       if (fe->vma->vm_flags & VM_SHARED) {
+               /* Clear PMD */
+               zap_page_range_single(fe->vma, fe->address,
+                               HPAGE_PMD_SIZE, NULL);
+               VM_BUG_ON(!pmd_none(*fe->pmd));
+
+               /* Refault to establish writable PMD */
+               return 0;
+       }
+
        /* COW handled on pte level: split pmd */
-       VM_BUG_ON_VMA(fe->vma->vm_flags & VM_SHARED, fe->vma);
        split_huge_pmd(fe->vma, fe->pmd, fe->address);
 
        return VM_FAULT_FALLBACK;
-- 
2.9.3