From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756880AbZCLP0G (ORCPT ); Thu, 12 Mar 2009 11:26:06 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753841AbZCLPZy (ORCPT ); Thu, 12 Mar 2009 11:25:54 -0400 Received: from waste.org ([66.93.16.53]:44037 "EHLO waste.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753293AbZCLPZx (ORCPT ); Thu, 12 Mar 2009 11:25:53 -0400 Subject: Re: [PATCH] acquire mmap semaphore in pagemap_read. From: Matt Mackall To: Martin Schwidefsky Cc: Alexey Dobriyan , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Gerald Schaefer , akpm@linux-foundation.org In-Reply-To: <20090312125410.25400d18@skybase> References: <20090312113308.6fe18a93@skybase> <20090312114533.GA2407@x200.localdomain> <20090312125410.25400d18@skybase> Content-Type: text/plain Date: Thu, 12 Mar 2009 10:23:34 -0500 Message-Id: <1236871414.3213.50.camel@calx> Mime-Version: 1.0 X-Mailer: Evolution 2.22.3.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2009-03-12 at 12:54 +0100, Martin Schwidefsky wrote: > On Thu, 12 Mar 2009 14:45:33 +0300 > Alexey Dobriyan wrote: > > > On Thu, Mar 12, 2009 at 11:33:08AM +0100, Martin Schwidefsky wrote: > > > --- linux-2.6/fs/proc/task_mmu.c > > > +++ linux-2.6-patched/fs/proc/task_mmu.c > > > @@ -716,7 +716,9 @@ static ssize_t pagemap_read(struct file > > > * user buffer is tracked in "pm", and the walk > > > * will stop when we hit the end of the buffer. > > > */ > > > + down_read(&mm->mmap_sem); > > > ret = walk_page_range(start_vaddr, end_vaddr, &pagemap_walk); > > > + up_read(&mm->mmap_sem); > > > > This will introduce "put_user under mmap_sem" which is deadlockable. > > Hmm, interesting. In this case the pagemap interface is fundamentally broken. Well it means we may have to reintroduce the very annoying double buffering from various earlier implementations. But let's leave this discussion until after we've figured out what to do about the walker code. -- http://selenic.com : development and support for Mercurial and Linux