From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Morton Subject: Re: msync(2) bug(?), returns AOP_WRITEPAGE_ACTIVATE to userland Date: Thu, 11 Oct 2007 14:47:40 -0700 Message-ID: <20071011144740.136b31a8.akpm@linux-foundation.org> References: <200710071920.l97JKJX5018871@agora.fsl.cs.sunysb.edu> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, ryan@finnie.org, cjwatson@ubuntu.com, linux-mm@kvack.org To: Erez Zadok Return-path: Received: from smtp2.linux-foundation.org ([207.189.120.14]:58708 "EHLO smtp2.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756036AbXJKVsC (ORCPT ); Thu, 11 Oct 2007 17:48:02 -0400 In-Reply-To: <200710071920.l97JKJX5018871@agora.fsl.cs.sunysb.edu> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On Sun, 7 Oct 2007 15:20:19 -0400 Erez Zadok wrote: > According to vfs.txt, ->writepage() may return AOP_WRITEPAGE_ACTIVATE back > to the VFS/VM. Indeed some filesystems such as tmpfs can return > AOP_WRITEPAGE_ACTIVATE; and stackable file systems (e.g., Unionfs) also > return AOP_WRITEPAGE_ACTIVATE if the lower f/s returned it. > > Anyway, some Ubuntu users of Unionfs reported that msync(2) sometimes > returns AOP_WRITEPAGE_ACTIVATE (decimal 524288) back to userland. > Therefore, some user programs fail, esp. if they're written such as this: > > err = msync(...); > if (err != 0) > // fail > > They temporarily fixed the specific program in question (apt-get) to check > > if (err < 0) > // fail > > Is this a bug indeed, or are user programs supposed to handle > AOP_WRITEPAGE_ACTIVATE (I hope not the latter). If it's a kernel bug, what > should the kernel return: a zero, or an -errno (and which one)? > shit. That's a nasty bug. Really userspace should be testing for -1, but the msync() library function should only ever return 0 or -1. Does this fix it? --- a/mm/page-writeback.c~a +++ a/mm/page-writeback.c @@ -850,8 +850,10 @@ retry: ret = (*writepage)(page, wbc, data); - if (unlikely(ret == AOP_WRITEPAGE_ACTIVATE)) + if (unlikely(ret == AOP_WRITEPAGE_ACTIVATE)) { unlock_page(page); + ret = 0; + } if (ret || (--(wbc->nr_to_write) <= 0)) done = 1; if (wbc->nonblocking && bdi_write_congested(bdi)) { _