From patchwork Mon Jun 12 12:23:04 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Jeff Layton X-Patchwork-Id: 774564 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 3wmXCz0qhMz9s71 for ; Mon, 12 Jun 2017 22:25:19 +1000 (AEST) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752423AbdFLMXq (ORCPT ); Mon, 12 Jun 2017 08:23:46 -0400 Received: from mx1.redhat.com ([209.132.183.28]:40566 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752351AbdFLMXl (ORCPT ); Mon, 12 Jun 2017 08:23:41 -0400 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id A64CE80C18; Mon, 12 Jun 2017 12:23:35 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com A64CE80C18 Authentication-Results: ext-mx02.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx02.extmail.prod.ext.phx2.redhat.com; spf=pass smtp.mailfrom=jlayton@redhat.com DKIM-Filter: OpenDKIM Filter v2.11.0 mx1.redhat.com A64CE80C18 Received: from tleilax.poochiereds.net (ovpn-120-91.rdu2.redhat.com [10.10.120.91]) by smtp.corp.redhat.com (Postfix) with ESMTP id 62C8882F80; Mon, 12 Jun 2017 12:23:34 +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: 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 v6 11/13] Documentation: flesh out the section in vfs.txt on storing and reporting writeback errors Date: Mon, 12 Jun 2017 08:23:04 -0400 Message-Id: <20170612122316.13244-13-jlayton@redhat.com> In-Reply-To: <20170612122316.13244-1-jlayton@redhat.com> References: <20170612122316.13244-1-jlayton@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.26]); Mon, 12 Jun 2017 12:23:36 +0000 (UTC) Sender: linux-ext4-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org I waxed a little loquacious here, but I figured that more detail was better, and writeback error handling is so hard to get right. Although I think we'll eventually remove it once the transition is complete, I've gone ahead and documented the FS_WB_ERRSEQ flag as well. Cc: Jan Kara Signed-off-by: Jeff Layton --- Documentation/filesystems/vfs.txt | 50 ++++++++++++++++++++++++++++++++++++--- 1 file changed, 47 insertions(+), 3 deletions(-) diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt index f42b90687d40..c3efdd833a3d 100644 --- a/Documentation/filesystems/vfs.txt +++ b/Documentation/filesystems/vfs.txt @@ -576,7 +576,49 @@ 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, expect that error to be +reported when fsync is called. After an error has been reported to +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 +filemap_set_wb_err 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). + +Older kernels used a different method for tracking errors, based on flags +in the address_space. We're currently switching everything over to use +the infrastructure based on errseq_t values. During the transition, +filesystem authors will want to also ensure their file_system_type has +FS_WB_ERRSEQ set in fs_flags to ensure that shared infrastructure is +aware of the model in use. struct address_space_operations ------------------------------- @@ -804,7 +846,8 @@ 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 +930,8 @@ 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