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