From patchwork Fri Jun 16 19:34:19 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Jeff Layton X-Patchwork-Id: 9793633 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork.web.codeaurora.org (Postfix) with ESMTP id 19D5E6038F for ; Fri, 16 Jun 2017 19:38:09 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 0A488285D1 for ; Fri, 16 Jun 2017 19:38:09 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id F29392863A; Fri, 16 Jun 2017 19:38:08 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.9 required=2.0 tests=BAYES_00,RCVD_IN_DNSWL_HI autolearn=unavailable version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 9ED2D28630 for ; Fri, 16 Jun 2017 19:38:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752645AbdFPTiF (ORCPT ); Fri, 16 Jun 2017 15:38:05 -0400 Received: from mx1.redhat.com ([209.132.183.28]:36488 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752541AbdFPTfv (ORCPT ); Fri, 16 Jun 2017 15:35:51 -0400 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 3F2B0C0587D7; Fri, 16 Jun 2017 19:35:46 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com 3F2B0C0587D7 Authentication-Results: ext-mx08.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx08.extmail.prod.ext.phx2.redhat.com; spf=pass smtp.mailfrom=jlayton@redhat.com DKIM-Filter: OpenDKIM Filter v2.11.0 mx1.redhat.com 3F2B0C0587D7 Received: from tleilax.poochiereds.net (ovpn-120-225.rdu2.redhat.com [10.10.120.225]) by smtp.corp.redhat.com (Postfix) with ESMTP id 8380C91427; Fri, 16 Jun 2017 19:35:44 +0000 (UTC) From: Jeff Layton To: Andrew Morton , Al Viro , Jan Kara , tytso@mit.edu, axboe@kernel.dk, mawilcox@microsoft.com, ross.zwisler@linux.intel.com, corbet@lwn.net, Chris Mason , Josef Bacik , David Sterba , "Darrick J . Wong" Cc: Carlos Maiolino , Eryu Guan , David Howells , Christoph Hellwig , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org, linux-btrfs@vger.kernel.org, linux-block@vger.kernel.org Subject: [PATCH v7 14/22] Documentation: flesh out the section in vfs.txt on storing and reporting writeback errors Date: Fri, 16 Jun 2017 15:34:19 -0400 Message-Id: <20170616193427.13955-15-jlayton@redhat.com> In-Reply-To: <20170616193427.13955-1-jlayton@redhat.com> References: <20170616193427.13955-1-jlayton@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.32]); Fri, 16 Jun 2017 19:35:46 +0000 (UTC) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP Let's try to make this extra clear for fs authors. Cc: Jan Kara Signed-off-by: Jeff Layton --- Documentation/filesystems/vfs.txt | 43 ++++++++++++++++++++++++++++++++++++--- 1 file changed, 40 insertions(+), 3 deletions(-) diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt index f42b90687d40..f3702d5c6f2f 100644 --- a/Documentation/filesystems/vfs.txt +++ b/Documentation/filesystems/vfs.txt @@ -576,7 +576,42 @@ should clear PG_Dirty and set PG_Writeback. It can be actually written at any point after PG_Dirty is clear. Once it is known to be safe, PG_Writeback is cleared. -Writeback makes use of a writeback_control structure... +Writeback makes use of a writeback_control structure to direct the +operations. This gives the the writepage and writepages operations some +information about the nature of and reason for the writeback request, +and the constraints under which it is being done. It is also used to +return information back to the caller about the result of a writepage or +writepages request. + +Handling errors during writeback +-------------------------------- +Most applications that utilize the pagecache will periodically call +fsync to ensure that data written has made it to the backing store. +When there is an error during writeback, they expect that error to be +reported when fsync is called. After an error has been reported on one +fsync, subsequent fsync calls on the same file descriptor should return +0, unless further writeback errors have occurred since the previous +fsync. + +Ideally, the kernel would report an error only on file descriptions on +which writes were done that subsequently failed to be written back. The +generic pagecache infrastructure does not track the file descriptions +that have dirtied each individual page however, so determining which +file descriptors should get back an error is not possible. + +Instead, the generic writeback error tracking infrastructure in the +kernel settles for reporting errors to fsync on all file descriptions +that were open at the time that the error occurred. In a situation with +multiple writers, all of them will get back an error on a subsequent fsync, +even if all of the writes done through that particular file descriptor +succeeded (or even if there were no writes on that file descriptor at all). + +Filesystems that wish to use this infrastructure should call +mapping_set_error to record the error in the address_space when it +occurs. Then, at the end of their fsync operation, they should call +filemap_report_wb_err to ensure that the struct file's error cursor +has advanced to the correct point in the stream of errors emitted by +the backing device(s). struct address_space_operations ------------------------------- @@ -804,7 +839,8 @@ struct address_space_operations { The File Object =============== -A file object represents a file opened by a process. +A file object represents a file opened by a process. This is also known +as an "open file description" in POSIX parlance. struct file_operations @@ -887,7 +923,8 @@ otherwise noted. release: called when the last reference to an open file is closed - fsync: called by the fsync(2) system call + fsync: called by the fsync(2) system call. Also see the section above + entitled "Handling errors during writeback". fasync: called by the fcntl(2) system call when asynchronous (non-blocking) mode is enabled for a file