From patchwork Thu Jun 29 13:19:48 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Jeff Layton X-Patchwork-Id: 782254 Return-Path: X-Original-To: patchwork-incoming@ozlabs.org Delivered-To: patchwork-incoming@ozlabs.org Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by ozlabs.org (Postfix) with ESMTP id 3wz0gp620Dz9s3T for ; Thu, 29 Jun 2017 23:22:14 +1000 (AEST) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753283AbdF2NWF (ORCPT ); Thu, 29 Jun 2017 09:22:05 -0400 Received: from mail.kernel.org ([198.145.29.99]:60252 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752845AbdF2NUe (ORCPT ); Thu, 29 Jun 2017 09:20:34 -0400 Received: from tleilax.poochiereds.net (cpe-45-37-196-243.nc.res.rr.com [45.37.196.243]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 9BE0F22BE1; Thu, 29 Jun 2017 13:20:26 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9BE0F22BE1 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=jlayton@kernel.org From: jlayton@kernel.org 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 , Liu Bo , 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 v8 12/18] Documentation: flesh out the section in vfs.txt on storing and reporting writeback errors Date: Thu, 29 Jun 2017 09:19:48 -0400 Message-Id: <20170629131954.28733-13-jlayton@kernel.org> X-Mailer: git-send-email 2.13.0 In-Reply-To: <20170629131954.28733-1-jlayton@kernel.org> References: <20170629131954.28733-1-jlayton@kernel.org> Sender: linux-ext4-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org From: Jeff Layton 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..1366043b3942 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 +file_check_and_advance_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