[PATCHv4 11/43] 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.shutemov@xxxxxxxxxxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
---
 mm/memory.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index e18c57bdc75c..e8c04d1d87b8 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3452,8 +3452,17 @@ 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_MASK,
+				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