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Documentation: flesh out the section in vfs.txt on storing and reporting writeback errors

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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>
This commit is contained in:
Jeff Layton 2017-07-06 07:02:27 -04:00
parent 8ed1e46aaf
commit acbf3c3452
1 changed files with 41 additions and 3 deletions
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44
Documentation/filesystems/vfs.txt
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@ -576,7 +576,43 @@ 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
-------------------------------
@ -804,7 +840,8 @@ 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 +924,8 @@ 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