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SubjectRe: [PATCHv9-rebased2 05/37] khugepaged: recheck pmd after mmap_sem re-acquired
On Wed, Jun 15, 2016 at 11:06:10PM +0300, Kirill A. Shutemov wrote:
> Vlastimil noted[1] that pmd can be no longer valid after we drop
> mmap_sem. We need recheck it once mmap_sem taken again.
>
> [1] http://lkml.kernel.org/r/12918dcd-a695-c6f4-e06f-69141c5f357f@suse.cz
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
> mm/huge_memory.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index d7ccc8558187..0efdad975659 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2437,6 +2437,9 @@ static bool __collapse_huge_page_swapin(struct mm_struct *mm,
> /* vma is no longer available, don't continue to swapin */
> if (hugepage_vma_revalidate(mm, address))
> return false;
> + /* check if the pmd is still valid */
> + if (mm_find_pmd(mm, address) != pmd)
> + return false;
> }
Thanks for fixing this.

> if (ret & VM_FAULT_ERROR) {
> trace_mm_collapse_huge_page_swapin(mm, swapped_in, 0);
> @@ -2522,6 +2525,9 @@ static void collapse_huge_page(struct mm_struct *mm,
> result = hugepage_vma_revalidate(mm, address);
> if (result)
> goto out;
> + /* check if the pmd is still valid */
> + if (mm_find_pmd(mm, address) != pmd)
> + goto out;
>
However here, I don't know do we need to check pmd.
Because in collapse_huge_page; pmd is newly created,
after taking mmap_sem read:
{

pmd_t *pmd, _pmd;
pte_t *pte;
...
down_read(&mm->mmap_sem);
result = hugepage_vma_revalidate(mm, address);
...
pmd = mm_find_pmd(mm, address);
...

Therefore it did not seem like a problem for me.

> anon_vma_lock_write(vma->anon_vma);
>
> --
> 2.8.1
>

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