From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk1-f200.google.com (mail-qk1-f200.google.com [209.85.222.200]) by kanga.kvack.org (Postfix) with ESMTP id 9BFCC8E0001 for ; Mon, 28 Jan 2019 08:14:37 -0500 (EST) Received: by mail-qk1-f200.google.com with SMTP id d196so18053968qkb.6 for ; Mon, 28 Jan 2019 05:14:37 -0800 (PST) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTPS id b8si2614567qvl.144.2019.01.28.05.14.36 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 28 Jan 2019 05:14:36 -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> From: David Hildenbrand Message-ID: Date: Mon, 28 Jan 2019 14:14:28 +0100 MIME-Version: 1.0 In-Reply-To: <20190128130709.GJ18811@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit 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 14:07, 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 balloon_page_insert() and balloon_page_delete() modify PageMovable and are only called with both, the page locked and b_dev_info->pages_lock locked. Especially: 1. balloon_page_dequeue() which tries to find an unlocked page that is not isolated. and deletes it via balloon_page_delete(). 2. balloon_page_migrate() will effectively call balloon_page_delete() on old page and balloon_page_insert() newpage. Both pages are locked before calling balloon_page_migrate() and the b_dev_info->pages_lock is taken internally. So what's left is a very small race window when migrating to a new page, after dropping the lock of the newpage. -- Thanks, David / dhildenb