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-rw-r--r--Documentation/filesystems/vfs.txt44
1 files changed, 41 insertions, 3 deletions
diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt
index f42b90687d40..48c9faa73a76 100644
--- a/Documentation/filesystems/vfs.txt
+++ b/Documentation/filesystems/vfs.txt
@@ -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