From mboxrd@z Thu Jan 1 00:00:00 1970 From: Hugh Dickins Subject: Re: msync(2) bug(?), returns AOP_WRITEPAGE_ACTIVATE to userland Date: Mon, 22 Oct 2007 21:01:11 +0100 (BST) Message-ID: References: <200710142232.l9EMW8kK029572@agora.fsl.cs.sunysb.edu> Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Cc: Pekka J Enberg , Ryan Finnie , Andrew Morton , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, cjwatson@ubuntu.com, linux-mm@kvack.org To: Erez Zadok Return-path: Received: from extu-mxob-2.symantec.com ([216.10.194.135]:34585 "EHLO extu-mxob-2.symantec.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751681AbXJVUCO (ORCPT ); Mon, 22 Oct 2007 16:02:14 -0400 Received: from l-036148a.enterprise.veritas.com([10.10.97.179]) (2893 bytes) by megami.veritas.com via sendmail with P:esmtp/R:smart_host/T:smtp (sender: ) id for ; Mon, 22 Oct 2007 13:01:23 -0700 (PDT) (Smail-3.2.0.101 1997-Dec-17 #15 built 2001-Aug-30) In-Reply-To: <200710142232.l9EMW8kK029572@agora.fsl.cs.sunysb.edu> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On Sun, 14 Oct 2007, Erez Zadok wrote: > In message , Pekka J Enberg writes: > > > > Look at mm/filemap.c:__filemap_fdatawrite_range(). You shouldn't be > > calling unionfs_writepage() _at all_ if the lower mapping has > > BDI_CAP_NO_WRITEBACK capability set. Perhaps something like the totally > > untested patch below? ... I don't disagree with your unionfs_writepages patch, Pekka, but I think it should be viewed as an optimization (don't waste time trying to write a group of pages when we know that nothing will be done) rather than as essential. Prior to unionfs's own use of AOP_WRITEPAGE_ACTIVATE, there have only been ramdisk and shmem generating it. ramdisk is careful only to return it in the wbc->for_reclaim case: I think (as in the patch I sent out before) shmem now ought to do so too for safety. Back in 2.4 days it was reasonable to assume that ->writepage would only get called from certain places, but things move faster nowadays, and the unionfs example shows others are liable to start ab/using it. I'll send Andrew that patch tomorrow (it's simple enough, but I'd like at least to try to reproduce the page_mapped bug first). > > Pekka, with a small change to your patch (to handle time-based cache > coherency), your patch worked well and passed all my tests. Thanks. > > So now I wonder if we still need the patch to prevent AOP_WRITEPAGE_ACTIVATE > from being returned to userland. I guess we still need it, b/c even with > your patch, generic_writepages() can return AOP_WRITEPAGE_ACTIVATE back to > the VFS and we need to ensure that doesn't "leak" outside the kernel. Can it now? Current git has a patch from Andrew which bears a striking resemblance to that from Pekka, stopping the leak from write_cache_pages. Hugh