Re: [PATCH v8 12/18] Documentation: flesh out the section in vfs.txt on storing and reporting writeback errors

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, Jun 29, 2017 at 09:19:48AM -0400, jlayton@kernel.org wrote:
> From: Jeff Layton <jlayton@redhat.com>
> 
> Let's try to make this extra clear for fs authors.
> 
> Cc: Jan Kara <jack@suse.cz>
> Signed-off-by: Jeff Layton <jlayton@redhat.com>
> ---
>  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
> 
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