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SubjectRe: [PATCHv9-rebased2 05/37] khugepaged: recheck pmd after mmap_sem re-acquired
On Thu, Jun 16, 2016 at 02:47:14PM +0300, Ebru Akagunduz wrote:
> 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 Vlastimil noted[1] that pmd can be no longer valid after we drop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.

I guess you're looking on the first hugepage_vma_revalidate() in
collapse_huge_page(). The patch fixes issue after the second one:

pmd_t *pmd, _pmd;
pte_t *pte;
...
down_read(&mm->mmap_sem);
result = hugepage_vma_revalidate(mm, address);
...
pmd = mm_find_pmd(mm, address);
...
up_read(&mm->mmap_sem);
...
down_write(&mm->mmap_sem);
result = hugepage_vma_revalidate(mm, address);
if (mm_find_pmd(mm, address) != pmd)
goto out;


--
Kirill A. Shutemov

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