From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758295AbcEFH3n (ORCPT ); Fri, 6 May 2016 03:29:43 -0400 Received: from mail-lf0-f48.google.com ([209.85.215.48]:36271 "EHLO mail-lf0-f48.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758204AbcEFH3k (ORCPT ); Fri, 6 May 2016 03:29:40 -0400 Date: Fri, 6 May 2016 10:29:36 +0300 From: "Kirill A. Shutemov" To: Andrea Arcangeli Cc: Alex Williamson , kirill.shutemov@linux.intel.com, linux-kernel@vger.kernel.org, "linux-mm@kvack.org" Subject: Re: [BUG] vfio device assignment regression with THP ref counting redesign Message-ID: <20160506072936.GA6971@node.shutemov.name> References: <20160429070611.GA4990@node.shutemov.name> <20160429163444.GM11700@redhat.com> <20160502104119.GA23305@node.shutemov.name> <20160502152307.GA12310@redhat.com> <20160502160042.GC24419@node.shutemov.name> <20160502180307.GB12310@redhat.com> <20160504191927.095cdd90@t450s.home> <20160505143924.GC28755@redhat.com> <20160505151110.GA13972@node.shutemov.name> <20160505152406.GH28755@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160505152406.GH28755@redhat.com> User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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