From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752151Ab1A0QYc (ORCPT ); Thu, 27 Jan 2011 11:24:32 -0500 Received: from bombadil.infradead.org ([18.85.46.34]:49012 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751223Ab1A0QYb convert rfc822-to-8bit (ORCPT ); Thu, 27 Jan 2011 11:24:31 -0500 Subject: Re: One (possible) x86 get_user_pages bug From: Peter Zijlstra To: Jan Beulich Cc: Xiaowei Yang , Nick Piggin , fanhenglong@huawei.com, Kaushik Barde , Kenneth Lee , linqaingmin , wangzhenguo@huawei.com, Wu Fengguang , "xen-devel@lists.xensource.com" , linux-kernel@vger.kernel.org In-Reply-To: <4D41A651020000780002ED36@vpn.id2.novell.com> References: <4D416D9A.9010603@huawei.com> <4D41A651020000780002ED36@vpn.id2.novell.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Date: Thu, 27 Jan 2011 17:25:02 +0100 Message-ID: <1296145502.15234.235.camel@laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.30.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2011-01-27 at 16:07 +0000, Jan Beulich wrote: > > Nick, based on your doing of the initial implementation, would > you be able to estimate whether disabling get_user_pages_fast() > altogether for Xen would be performing measurably worse than > adding the locks (but continuing to avoid acquiring mm->mmap_sem) > as suggested by Xiaowei? That's of course only if the latter is correct > at all, of which I haven't fully convinced myself yet. Adding the lock will result in deadlocks, __get_user_pages_fast() is used from NMI context. Then again, I've got no clue if Xen even has NMIs..