mm, dax: check for pmd_none() after split_huge_pmd()

DAX implements split_huge_pmd() by clearing pmd. This simple approach reduces memory overhead, as we don't need to deposit page table on huge page mapping to make split_huge_pmd() never-fail. PTE table can be allocated and populated later on page fault from backing store. But one side effect is that have to check if pmd is pmd_none() after split_huge_pmd(). In most places we do this already to deal with parallel MADV_DONTNEED. But I found two call sites which is not affected by MADV_DONTNEED (due down_write(mmap_sem)), but need to have the check to work with DAX properly. Signed-off-by: Kirill A. Shutemov <[email protected]> Cc: Dan Williams <[email protected]> Cc: Matthew Wilcox <[email protected]> Cc: Andrea Arcangeli <[email protected]> Cc: Ross Zwisler <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]>
josf · Feb 12, 2016 · 6b9116a · 6b9116a
1 parent 7eb3912
commit 6b9116a
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Showing 2 changed files with 6 additions and 2 deletions.
diff --git a/mm/mprotect.c b/mm/mprotect.c
@@ -160,9 +160,11 @@ static inline unsigned long change_pmd_range(struct vm_area_struct *vma,
 		}
 
 		if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd)) {
-			if (next - addr != HPAGE_PMD_SIZE)
+			if (next - addr != HPAGE_PMD_SIZE) {
 				split_huge_pmd(vma, pmd, addr);
-			else {
+				if (pmd_none(*pmd))
+					continue;
+			} else {
 				int nr_ptes = change_huge_pmd(vma, pmd, addr,
 						newprot, prot_numa);
 

diff --git a/mm/mremap.c b/mm/mremap.c
@@ -210,6 +210,8 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
 				}
 			}
 			split_huge_pmd(vma, old_pmd, old_addr);
+			if (pmd_none(*old_pmd))
+				continue;
 			VM_BUG_ON(pmd_trans_huge(*old_pmd));
 		}
 		if (pmd_none(*new_pmd) && __pte_alloc(new_vma->vm_mm, new_vma,