Re: [BUG] vfio device assignment regression with THP ref counting redesign

Kirill A. Shutemov
2016-05-06T07:29:43Z

On Thu, May 05, 2016 at 05:24:06PM +0200, Andrea Arcangeli wrote:
> On Thu, May 05, 2016 at 06:11:10PM +0300, Kirill A. Shutemov wrote:
> > Hm. How total_mapcount equal to NULL wouldn't lead to NULL-pointer
> > dereference inside page_trans_huge_mapcount()?
>
> Sorry for the confusion, this was still work in progress and then I've
> seen the email from Alex and I sent the last version I had committed
> right away. An earlier version of course had the proper checks for
> NULL but they got wiped as I transitioned from one model to another
> and back.
>
> > > + page_move_anon_rmap(old_page, vma, address);
> >
> > compound_head() is missing, I believe.
>
> Oh yes, fixed that too.
>
> if (total_mapcount == 1) {
> /*
> * The page is all ours. Move it to
> * our anon_vma so the rmap code will
> * not search our parent or siblings.
> * Protected against the rmap code by
> * the page lock.
> */
> page_move_anon_rmap(compound_head(old_page),
> vma, address);
> }
>
>
> If there's no other issue I can git send-email.

I don't see any.

> Then we should look into calling page_move_anon_rmap from THP COWs
> too, hugetlbfs calls it too. I think we probably need to make
> page_move_anon_rmap smarter and optionally let it take the lock for us
> after reading page->mapping first to be sure it's really moving it.
>
> The question is then if trylock or lock_page should be used, my
> preference would be just trylock.

trylock is probably fine. It's not big deal if we wouldn't move the page
to new anon_vma, just nice-to-have.

--
Kirill A. Shutemov