From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Pekka Enberg" Subject: Re: msync(2) bug(?), returns AOP_WRITEPAGE_ACTIVATE to userland Date: Sun, 14 Oct 2007 20:09:34 +0300 Message-ID: <84144f020710141009xbc5bb71w64e8288f364ab491@mail.gmail.com> References: <200710071920.l97JKJX5018871@agora.fsl.cs.sunysb.edu> <20071011144740.136b31a8.akpm@linux-foundation.org> <84144f020710121445p23fcc21am18482e01856cdc35@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: "Ryan Finnie" , "Andrew Morton" , "Erez Zadok" , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, cjwatson@ubuntu.com, linux-mm@kvack.org To: "Hugh Dickins" Return-path: Received: from rv-out-0910.google.com ([209.85.198.188]:35324 "EHLO rv-out-0910.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758152AbXJNRJj (ORCPT ); Sun, 14 Oct 2007 13:09:39 -0400 Received: by rv-out-0910.google.com with SMTP id k20so1232119rvb for ; Sun, 14 Oct 2007 10:09:39 -0700 (PDT) In-Reply-To: Content-Disposition: inline Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org Hi Hugh, On Sat, 13 Oct 2007, Pekka Enberg wrote: > Doesn't msync(2) get to it via mm/page-writeback.c:write_cache_pages() > without unionfs even? On 10/14/07, Hugh Dickins wrote: > I believe not. Please do double-check my assertions, I've always found > the _writepages paths rather twisty; but my belief (supported by the > fact that we've not hit shmem_writepage's BUG_ON(page_mapped(page)) > in five years) is that tmpfs/shmem opts out of all of that with its > .capabilities = BDI_CAP_NO_ACCT_DIRTY | BDI_CAP_NO_WRITEBACK, > in shmem_backing_dev_info, which avoids all those _writepages avenues > (e.g. through bdi_cap_writeback_dirty tests), and write_cache_pages is > just a subfunction of the _writepages. Thanks for the explanation, you're obviously correct. However, I don't think the mapping_cap_writeback_dirty() check in __filemap_fdatawrite_range() works as expected when tmpfs is a lower mount for an unionfs mount. There's no BDI_CAP_NO_WRITEBACK capability for unionfs mappings so do_fsync() will call write_cache_pages() that unconditionally invokes shmem_writepage() via unionfs_writepage(). Unless, of course, there's some other unionfs magic I am missing. Pekka