From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qt1-f197.google.com (mail-qt1-f197.google.com [209.85.160.197]) by kanga.kvack.org (Postfix) with ESMTP id 9B7228E0001 for ; Mon, 28 Jan 2019 09:38:52 -0500 (EST) Received: by mail-qt1-f197.google.com with SMTP id w1so20468100qta.12 for ; Mon, 28 Jan 2019 06:38:52 -0800 (PST) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTPS id s16si1158403qtq.248.2019.01.28.06.38.51 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 28 Jan 2019 06:38:51 -0800 (PST) Subject: Re: [PATCH RFC] mm: migrate: don't rely on PageMovable() of newpage after unlocking it 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> From: David Hildenbrand Message-ID: Date: Mon, 28 Jan 2019 15:38:38 +0100 MIME-Version: 1.0 In-Reply-To: <20190128133514.GL18811@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko 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 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. -- Thanks, David / dhildenb