From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Morton Subject: Re: [patch 9/9] mm: fix pagecache write deadlocks Date: Sun, 4 Feb 2007 03:15:49 -0800 Message-ID: <20070204031549.203f7b47.akpm@linux-foundation.org> References: <20070204063707.23659.20741.sendpatchset@linux.site> <20070204063833.23659.55105.sendpatchset@linux.site> <20070204014445.88e6c8c7.akpm@linux-foundation.org> <20070204101529.GA22004@wotan.suse.de> <20070204023055.2583fd65.akpm@linux-foundation.org> <20070204104609.GA29943@wotan.suse.de> <20070204025602.a5f8c53a.akpm@linux-foundation.org> <20070204110317.GA9034@wotan.suse.de> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: Linux Kernel , Linux Filesystems , Linux Memory Management To: Nick Piggin Return-path: Received: from smtp.osdl.org ([65.172.181.24]:37037 "EHLO smtp.osdl.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752303AbXBDLQF (ORCPT ); Sun, 4 Feb 2007 06:16:05 -0500 In-Reply-To: <20070204110317.GA9034@wotan.suse.de> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On Sun, 4 Feb 2007 12:03:17 +0100 Nick Piggin wrote: > On Sun, Feb 04, 2007 at 02:56:02AM -0800, Andrew Morton wrote: > > On Sun, 4 Feb 2007 11:46:09 +0100 Nick Piggin wrote: > > > > > On Sun, Feb 04, 2007 at 02:30:55AM -0800, Andrew Morton wrote: > > > > On Sun, 4 Feb 2007 11:15:29 +0100 Nick Piggin wrote: > > > > > > > > > The write path is broken. I prefer my kernels slow, than buggy. > > > > > > > > That won't fly. > > > > > > What won't fly? > > > > I suspect the performance cost of this approach would force us to redo it > > all. > > That's the idea. But at least in the meantime we're correct. There's no way I'd support merging a change which we know we'll have to redo only we have no clue how. > > If that recollection is right, I think we could afford to reintroduce that > > problem, frankly. Especially as it only happens in the incredibly rare > > case of that get_user()ed page getting unmapped under our feet. > > Dang. I was hoping to fix it without introducing data corruption. Well. It's a compromise. Being practical about it, I reeeealy doubt that anyone will hit this combination of circumstances. > > > > > but you introduce the theoretical memory deadlock > > > > > where a task cannot reclaim its own memory. > > > > > > > > Nah, that'll never happen - both pages are already allocated. > > > > > > Both pages? I don't get it. > > > > > > You set the don't-reclaim vma flag, then run get_user, which takes a > > > page fault and potentially has to allocate N pages for pagetables, > > > pagecache readahead, buffers and fs private data and pagecache radix > > > tree nodes for all of the pages read in. > > > > Oh, OK. Need to do the get_user() twice then. Once before taking that new > > rwsem. > > Race condition remains. No, not in a million years.