From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f72.google.com (mail-ed1-f72.google.com [209.85.208.72]) by kanga.kvack.org (Postfix) with ESMTP id 84E748E0001 for ; Mon, 28 Jan 2019 09:53:26 -0500 (EST) Received: by mail-ed1-f72.google.com with SMTP id c34so6563060edb.8 for ; Mon, 28 Jan 2019 06:53:26 -0800 (PST) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id w5-v6si1450070eja.270.2019.01.28.06.53.25 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 28 Jan 2019 06:53:25 -0800 (PST) Date: Mon, 28 Jan 2019 15:53:23 +0100 From: Michal Hocko Subject: Re: [PATCH RFC] mm: migrate: don't rely on PageMovable() of newpage after unlocking it Message-ID: <20190128145323.GN18811@dhcp22.suse.cz> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: David Hildenbrand Cc: 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 , Rafael Aquini On Mon 28-01-19 15:38:38, 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. Please make sure to CC Minchan when reposting. Btw. Acked-by: Michal Hocko -- Michal Hocko SUSE Labs