diff options
| -rw-r--r-- | Documentation/filesystems/vfs.txt | 44 |
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 |