Re: [PATCH, RFC 7/9] thp: implement splitting pmd for huge zero page

[Andrea Arcangeli <aarcange@redhat.com>]

On Fri, Aug 17, 2012 at 11:12:33AM +0300, Kirill A. Shutemov wrote:
> I've used do_huge_pmd_wp_page_fallback() as template for my code.
> What's difference between these two code paths?
> Why is do_huge_pmd_wp_page_fallback() safe?

Good point. do_huge_pmd_wp_page_fallback works only on the current
"mm" so it doesn't need the splitting transition, but thinking twice
the split_huge_zero_page_pmd also works only on the local "mm" because
you're not really splitting the zero page there (you're not affecting
other mm). As long as you keep holding the page_table_lock of the "mm"
that you're altering your current version is safe.

I got mistaken because I'm very used to think at split huge page as
something that cannot relay on the page_table_lock, but this is a
simpler case that isn't splitting the "page" but only the "pmd" of a
single "mm", so you can safely relay on the mm->page_table_lock :).

> Looks resonable. I'll update it in next revision.

Thanks. Of course the function parameter comments to avoid unnecessary
calls of find_vma, weren't related to the above locking issues.