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 <kirill.shutemov@linux.intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Matthew Wilcox <willy@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 6 insertions, 2 deletions

diff --git a/mm/mprotect.c b/mm/mprotect.c
index 8eb7bb40dc40..f7cb3d4d9c2e 100644
--- 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
index d77946a997f7..8eeba02fc991 100644
--- 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,