[Devel] [PATCH RHEL7 COMMIT] ms/Documentation: flesh out the section in vfs.txt on storing and reporting writeback errors

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-862.vz7.48.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-862.el7
------>
commit 590b8ecbc61ee212eaf9ee1a28330772a75c488b
Author: Vasily Averin <vvs at virtuozzo.com>
Date:   Sat Jun 9 13:29:36 2018 +0300

    ms/Documentation: flesh out the section in vfs.txt on storing and reporting writeback errors
    
    mainline commit acbf3c3 ("Documentation: flesh out the section in vfs.txt on storing and reporting writeback errors")
    
    Let's try to make this extra clear for fs authors.
    
    Cc: Jan Kara <jack at suse.cz>
    Signed-off-by: Jeff Layton <jlayton at redhat.com>
    Signed-off-by: Vasily Averin <vvs at virtuozzo.com>
---
 Documentation/filesystems/vfs.txt | 44 ++++++++++++++++++++++++++++++++++++---
 1 file changed, 41 insertions(+), 3 deletions(-)

diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt
index 8d5db43697a0..0acefb50736b 100644
--- a/Documentation/filesystems/vfs.txt
+++ b/Documentation/filesystems/vfs.txt
@@ -561,7 +561,43 @@ 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 do buffered I/O will periodically call a file
+synchronization call (fsync, fdatasync, msync or sync_file_range) to
+ensure that data written has made it to the backing store.  When there
+is an error during writeback, they expect that error to be reported when
+a file sync request is made.  After an error has been reported on one
+request, subsequent requests on the same file descriptor should return
+0, unless further writeback errors have occurred since the previous file
+syncronization.
+
+Ideally, the kernel would report errors 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, after writing back data from the pagecache in their
+file->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
 -------------------------------
@@ -778,7 +814,8 @@ 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
@@ -859,7 +896,8 @@ 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