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

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Message-ID: <20160506072936.GA6971@node.shutemov.name>
Date:	Fri, 6 May 2016 10:29:36 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Alex Williamson <alex.williamson@...hat.com>,
	kirill.shutemov@...ux.intel.com, linux-kernel@...r.kernel.org,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [BUG] vfio device assignment regression with THP ref counting
 redesign

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