From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Thu, 29 Jun 2017 10:11:37 -0700 From: "Darrick J. Wong" To: jlayton@kernel.org Cc: 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 , 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: Re: [PATCH v8 12/18] Documentation: flesh out the section in vfs.txt on storing and reporting writeback errors Message-ID: <20170629171137.GE5874@birch.djwong.org> References: <20170629131954.28733-1-jlayton@kernel.org> <20170629131954.28733-13-jlayton@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20170629131954.28733-13-jlayton@kernel.org> List-ID: On Thu, Jun 29, 2017 at 09:19:48AM -0400, jlayton@kernel.org wrote: > 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. /me wonders if this sentence ought to be worded more strongly, e.g. "Applications that utilize the pagecache must call a data synchronization syscall such as fsync, fdatasync, or msync to ensure that data written has made it to the backing store." I'm also wondering -- fdatasync and msync will also report any writeback errors that have happened anywhere (like fsync), since they all map to vfs_fsync_range, correct? If so, I think it worth it to state explicitly that the other *sync methods behave the same as fsync w.r.t. writeback error reporting. --D > +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 > -- > 2.13.0 > > -- > To unsubscribe from this list: send the line "unsubscribe linux-xfs" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html