Commit 6b9116a6 authored Feb 11, 2016 by Kirill A. Shutemov Committed by Linus Torvalds Feb 11, 2016

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>

parent 7eb39129

mm/mprotect.c

+4 −2

Original line number	Diff line number	Diff line
		@@ -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);

mm/mremap.c

+2 −0

Original line number	Diff line number	Diff line
		@@ -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,

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