From mboxrd@z Thu Jan 1 00:00:00 1970 From: Nick Piggin Subject: Re: [patch 9/9] mm: fix pagecache write deadlocks Date: Sun, 4 Feb 2007 11:46:09 +0100 Message-ID: <20070204104609.GA29943@wotan.suse.de> 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> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Linux Kernel , Linux Filesystems , Linux Memory Management To: Andrew Morton Return-path: Received: from mx1.suse.de ([195.135.220.2]:56404 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752265AbXBDKqL (ORCPT ); Sun, 4 Feb 2007 05:46:11 -0500 Content-Disposition: inline In-Reply-To: <20070204023055.2583fd65.akpm@linux-foundation.org> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org 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? > > > > There's a build error in filemap_xip.c btw. > > ? Thanks? > > > What happened to the idea of doing an atomic copy into the non-uptodate > > > page and handling it somehow? > > > > That was my second idea. > > Coulda sworn it was mine ;) I thought you ended up deciding it wasn't > practical because of the games we needed to play with ->commit_write. Maybe I misunderstood what you meant, above. I have an alterative fix where a temporary page is allocated if the write enncounters a non uptodate page. The usercopy then goes into that page, and from there into the target page after we have opened the prepare_write(). My *first* idea to fix this was to do the atomic copy into a non-uptodate page and then calling a zero-length commit_write if it failed. I pretty carefully constructed all these good arguments as to why each case works properly, but in the end it just didn't fly because it broke lots of filesystems. > > > Another option might be to effectively pin the whole mm during the copy: > > > > > > down_read(¤t->mm->unpaging_lock); > > > get_user(addr); /* Fault the page in */ > > > ... > > > copy_from_user() > > > up_read(¤t->mm->unpaging_lock); > > > > > > then, anyone who wants to unmap pages from this mm requires > > > write_lock(unpaging_lock). So we know the results of that get_user() > > > cannot be undone. > > > > Fugly. > > I invited you to think different - don't just fixate on one random > tossed-out-there suggestion. I've thought. Quite a lot. I have 2 other approaches that don't require mmap_sem, and 1 which is actually possible to implement without breaking filesystems. > > 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. > It's better than taking mmap_sem and walking pagetables... I'm not convinced.