From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754129AbdEIQYY (ORCPT ); Tue, 9 May 2017 12:24:24 -0400 Received: from mail-qk0-f172.google.com ([209.85.220.172]:33928 "EHLO mail-qk0-f172.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753105AbdEIQYU (ORCPT ); Tue, 9 May 2017 12:24:20 -0400 Message-ID: <1494347057.2659.10.camel@redhat.com> Subject: Re: [PATCH v4 25/27] Documentation: flesh out the section in vfs.txt on storing and reporting writeback errors From: Jeff Layton To: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-btrfs@vger.kernel.org, linux-ext4@vger.kernel.org, linux-cifs@vger.kernel.org, linux-nfs@vger.kernel.org, linux-mm@kvack.org, jfs-discussion@lists.sourceforge.net, linux-xfs@vger.kernel.org, cluster-devel@redhat.com, linux-f2fs-devel@lists.sourceforge.net, v9fs-developer@lists.sourceforge.net, linux-nilfs@vger.kernel.org, linux-block@vger.kernel.org Cc: dhowells@redhat.com, akpm@linux-foundation.org, hch@infradead.org, ross.zwisler@linux.intel.com, mawilcox@microsoft.com, jack@suse.com, viro@zeniv.linux.org.uk, corbet@lwn.net, neilb@suse.de, clm@fb.com, tytso@mit.edu, axboe@kernel.dk, josef@toxicpanda.com, hubcap@omnibond.com, rpeterso@redhat.com, bo.li.liu@oracle.com, Michael Kerrisk Date: Tue, 09 May 2017 12:24:17 -0400 In-Reply-To: <20170509154930.29524-26-jlayton@redhat.com> References: <20170509154930.29524-1-jlayton@redhat.com> <20170509154930.29524-26-jlayton@redhat.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.22.6 (3.22.6-2.fc25) Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2017-05-09 at 11:49 -0400, Jeff Layton wrote: > I waxed a little loquacious here, but I figured that more detail was > better, and writeback error handling is so hard to get right. > > Cc: Jan Kara > Signed-off-by: Jeff Layton > --- > Documentation/filesystems/vfs.txt | 54 ++++++++++++++++++++++++++++++++------- > 1 file changed, 45 insertions(+), 9 deletions(-) > > diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt > index f201a77873f7..382190a872e5 100644 > --- a/Documentation/filesystems/vfs.txt > +++ b/Documentation/filesystems/vfs.txt > @@ -576,12 +576,46 @@ 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. > > -If there is an error during writeback, then the address_space should be > -marked with an error (typically using mapping_set_error), in order to > -ensure that the error can later be reported to the application when an > -fsync is issued. > - > -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, that error should 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 it 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). > + (cc'ing Michael Kerrisk) Once this is closer to merge, I think we'll also want to update the fsync(2) manpage with something similar to the 3 paragraphs above, and also with an explanation of the behavior that applications can expect from earlier kernels. > +Filesystems that wish to use this infrastructure should call > +mapping_set_error to record the error in the address_space when it > +occurs. The generic vfs code will then handle reporting the error when > +fsync is called, even if the fsync file operation returned 0. > + > +Filesystems are free to track errors internally if they choose (i.e. if > +they do keep track of how the pages were dirtied), but they should aim > +to provide the same (or better) error reporting semantics for when there > +are multiple writers. Those filesystems should avoid calling > +mapping_set_error in order to ensure that errors stored in the mapping > +aren't improperly reported by the generic filesystem code. > > struct address_space_operations > ------------------------------- > @@ -810,7 +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 > @@ -893,9 +928,10 @@ otherwise noted. > > release: called when the last reference to an open file is closed > > - fsync: called by the fsync(2) system call. Errors that were previously > + fsync: called by the fsync(2) system call. Errors that were previously > recorded using mapping_set_error will automatically be returned to > - the application and the file's error sequence advanced. > + the application and the struct file's error sequence advanced. > + See the section above on handling writeback errors. > > fasync: called by the fcntl(2) system call when asynchronous > (non-blocking) mode is enabled for a file -- Jeff Layton