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

[Andrea Arcangeli]

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.

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.