thp: handle file COW faults

File COW for THP is handled on pte level: just split the pmd.

It's not clear how benefitial would be allocation of huge pages on COW
faults.  And it would require some code to make them work.

I think at some point we can consider teaching khugepaged to collapse
pages in COW mappings, but allocating huge on fault is probably
overkill.

Link: http://lkml.kernel.org/r/1466021202-61880-16-git-send-email-kirill.shutemov@linux.intel.com
Signed-off-by: Kirill A. Shutemov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

diff --git a/mm/memory.c b/mm/memory.c
index 10a424eca8a470c8c1bfb14ed1186be803c5fb3b..8f42547981306e0ad73b12ea5af490aa6fe523fd 100644 (file)
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3446,6 +3446,11 @@ static int wp_huge_pmd(struct fault_env *fe, pmd_t orig_pmd)
        if (fe->vma->vm_ops->pmd_fault)
                return fe->vma->vm_ops->pmd_fault(fe->vma, fe->address, fe->pmd,
                                fe->flags);
+
+       /* 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;
 }