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

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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]>
This commit is contained in:
Kirill A. Shutemov 2016-02-11 16:13:03 -08:00 • committed by Linus Torvalds
commit 6b9116a652
2 changed files with 6 additions and 2 deletions
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6
mm/mprotect.c
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@ -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);