From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk1-f198.google.com (mail-qk1-f198.google.com [209.85.222.198]) by kanga.kvack.org (Postfix) with ESMTP id 72A6F8E0001 for ; Mon, 28 Jan 2019 10:04:32 -0500 (EST) Received: by mail-qk1-f198.google.com with SMTP id k203so18546182qke.2 for ; Mon, 28 Jan 2019 07:04:32 -0800 (PST) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTPS id n127si699296qkf.230.2019.01.28.07.04.31 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 28 Jan 2019 07:04:31 -0800 (PST) Date: Mon, 28 Jan 2019 10:04:20 -0500 From: Rafael Aquini Subject: Re: [PATCH RFC] mm: migrate: don't rely on PageMovable() of newpage after unlocking it Message-ID: <20190128150420.GB10872@xps> References: <20190128121609.9528-1-david@redhat.com> <20190128130709.GJ18811@dhcp22.suse.cz> <20190128132146.GK18811@dhcp22.suse.cz> <17e7d7e4-f4ca-a681-93e5-92a0c285be14@redhat.com> <20190128133514.GL18811@dhcp22.suse.cz> <20190128150156.GA10872@xps> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190128150156.GA10872@xps> Sender: owner-linux-mm@kvack.org List-ID: To: David Hildenbrand Cc: Michal Hocko , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Mel Gorman , "Kirill A. Shutemov" , Naoya Horiguchi , Jan Kara , Andrea Arcangeli , Dominik Brodowski , Matthew Wilcox , Vratislav Bendel On Mon, Jan 28, 2019 at 10:01:56AM -0500, Rafael Aquini wrote: > On Mon, Jan 28, 2019 at 03:38:38PM +0100, David Hildenbrand wrote: > > On 28.01.19 14:35, Michal Hocko wrote: > > > On Mon 28-01-19 14:22:52, David Hildenbrand wrote: > > >> On 28.01.19 14:21, Michal Hocko wrote: > > >>> On Mon 28-01-19 14:14:28, David Hildenbrand wrote: > > >>>> On 28.01.19 14:07, Michal Hocko wrote: > > >>>>> On Mon 28-01-19 13:16:09, David Hildenbrand wrote: > > >>>>> [...] > > >>>>>> My theory: > > >>>>>> > > >>>>>> In __unmap_and_move(), we lock the old and newpage and perform the > > >>>>>> migration. In case of vitio-balloon, the new page will become > > >>>>>> movable, the old page will no longer be movable. > > >>>>>> > > >>>>>> However, after unlocking newpage, I think there is nothing stopping > > >>>>>> the newpage from getting dequeued and freed by virtio-balloon. This > > >>>>>> will result in the newpage > > >>>>>> 1. No longer having PageMovable() > > >>>>>> 2. Getting moved to the local list before finally freeing it (using > > >>>>>> page->lru) > > >>>>> > > >>>>> Does that mean that the virtio-balloon can change the Movable state > > >>>>> while there are other users of the page? Can you point to the code that > > >>>>> does it? How come this can be safe at all? Or is the PageMovable stable > > >>>>> only under the page lock? > > >>>>> > > >>>> > > >>>> PageMovable is stable under the lock. The relevant instructions are in > > >>>> > > >>>> mm/balloon_compaction.c and include/linux/balloon_compaction.h > > >>> > > >>> OK, I have just checked __ClearPageMovable and it indeed requires > > >>> PageLock. Then we also have to move is_lru = __PageMovable(page) after > > >>> the page lock. > > >>> > > >> > > >> I assume that is fine as is as the page is isolated? (yes, it will be > > >> modified later when moving but we are interested in the original state) > > > > > > OK, I've missed that the page is indeed isolated. Then the patch makes > > > sense to me. > > > > > > > Thanks Michal. I assume this has broken ever since balloon compaction > > was introduced. I'll wait a little more and then resend as !RFC with a > > cc-stable tag. > > > > Yes, balloon deflation could always race against migration > This race was a problem, initially, and was dealt with, via: > > commit 117aad1e9e4d97448d1df3f84b08bd65811e6d6a > Author: Rafael Aquini > Date: Mon Sep 30 13:45:16 2013 -0700 > > mm: avoid reinserting isolated balloon pages into LRU lists > > > > I think this upstream patch has re-introduced it, in a more subtle way, > as we're stumbling on it now, again: > > commit d6d86c0a7f8ddc5b38cf089222cb1d9540762dc2 > Author: Konstantin Khlebnikov > Date: Thu Oct 9 15:29:27 2014 -0700 > > mm/balloon_compaction: redesign ballooned pages management > > > > On this particular race against migration case, virtio ballon deflation would > not see it before > > commit b1123ea6d3b3da25af5c8a9d843bd07ab63213f4 > Author: Minchan Kim > Date: Tue Jul 26 15:23:09 2016 -0700 > > mm: balloon: use general non-lru movable page feature > > as the recently released balloon page would be post-processed > without the page->lru list handling, which for migration stability > purposes must be done under the protection of page_lock. > > missing part here: I think your patch adresses this new case. Acked-by: Rafael Aquini > get rid of balloon reference count. ^^ this was a left over (sorry about my fat-fingers) > > > -- Rafael