Andrea Arcangeli wrote:
> Hi Hugh,
>
> On Tue, Oct 15, 2013 at 04:08:28AM -0700, Hugh Dickins wrote:
> > Occasionally we hit the BUG_ON(pmd_trans_huge(*pmd)) at the end of
> > __split_huge_page_pmd(): seen when doing madvise(,,MADV_DONTNEED).
> >
> > It's invalid: we don't always have down_write of mmap_sem there:
> > a racing do_huge_pmd_wp_page() might have copied-on-write to another
> > huge page before our split_huge_page() got the anon_vma lock.
> >
>
> I don't get exactly the scenario with do_huge_pmd_wp_page(), could you
> elaborate?
I think the scenario is follow:
CPU0: CPU1
__split_huge_page_pmd()
page = pmd_page(*pmd);
do_huge_pmd_wp_page() copy the
page and changes pmd (the same as on
CPU0)
to point to newly copied page.
split_huge_page(page)
where page is original page,
not allocated on COW.
pmd still points on huge page.
Hugh, have I got it correctly?
--
Kirill A. Shutemov
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