From patchwork Mon Jun 12 12:23:09 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Jeff Layton X-Patchwork-Id: 9780891 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 F2EFF60244 for ; Mon, 12 Jun 2017 12:24:58 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id D99982848F for ; Mon, 12 Jun 2017 12:24:58 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id CE0EF284B1; Mon, 12 Jun 2017 12:24:58 +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 4386F2836F for ; Mon, 12 Jun 2017 12:24:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752477AbdFLMXv (ORCPT ); Mon, 12 Jun 2017 08:23:51 -0400 Received: from mx1.redhat.com ([209.132.183.28]:50972 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751966AbdFLMXs (ORCPT ); Mon, 12 Jun 2017 08:23:48 -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 C46FAC04B317; Mon, 12 Jun 2017 12:23:42 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com C46FAC04B317 Authentication-Results: ext-mx07.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx07.extmail.prod.ext.phx2.redhat.com; spf=pass smtp.mailfrom=jlayton@redhat.com DKIM-Filter: OpenDKIM Filter v2.11.0 mx1.redhat.com C46FAC04B317 Received: from tleilax.poochiereds.net (ovpn-120-91.rdu2.redhat.com [10.10.120.91]) by smtp.corp.redhat.com (Postfix) with ESMTP id 3FF6A82F80; Mon, 12 Jun 2017 12:23:41 +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 13/20] Documentation: flesh out the section in vfs.txt on storing and reporting writeback errors Date: Mon, 12 Jun 2017 08:23:09 -0400 Message-Id: <20170612122316.13244-18-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 [10.5.110.31]); Mon, 12 Jun 2017 12:23:43 +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. Also, 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 | 48 ++++++++++++++++++++++++++++++++++++--- 1 file changed, 45 insertions(+), 3 deletions(-) diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt index f42b90687d40..0f6415c26385 100644 --- a/Documentation/filesystems/vfs.txt +++ b/Documentation/filesystems/vfs.txt @@ -576,7 +576,47 @@ 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 need to do two things: + +1) call mapping_set_error to record the error in the address_space when +one occurs. + +2) set FS_WB_ERRSEQ in the fs_flags field in the file_system_type to +indicate to other subsystems that the filesystem wants to use errseq_t +based error reporting for writeback. + +The flag may go away in the future or moved to an opt-out flag once +the majority of filesystems are converted to use errseq_t based reporting. struct address_space_operations ------------------------------- @@ -804,7 +844,8 @@ 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 +928,8 @@ 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