[PATCH v6 13/20] Documentation: flesh out the section in vfs.txt on storing and reporting writeback errors

[Jeff Layton <jlayton@redhat.com>]

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 <jack@suse.cz>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
---
 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
-- 
2.13.0

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