| Message ID | 20170509154930.29524-26-jlayton@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Tue, 2017-05-09 at 11:49 -0400, Jeff Layton wrote: > I waxed a little loquacious here, but I figured that more detail was > better, and writeback error handling is so hard to get right. > > Cc: Jan Kara <jack@suse.cz> > Signed-off-by: Jeff Layton <jlayton@redhat.com> > --- > Documentation/filesystems/vfs.txt | 54 ++++++++++++++++++++++++++++++++------- > 1 file changed, 45 insertions(+), 9 deletions(-) > > diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt > index f201a77873f7..382190a872e5 100644 > --- a/Documentation/filesystems/vfs.txt > +++ b/Documentation/filesystems/vfs.txt > @@ -576,12 +576,46 @@ 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. > > -If there is an error during writeback, then the address_space should be > -marked with an error (typically using mapping_set_error), in order to > -ensure that the error can later be reported to the application when an > -fsync is issued. > - > -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, that error should 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 it 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). > + (cc'ing Michael Kerrisk) Once this is closer to merge, I think we'll also want to update the fsync(2) manpage with something similar to the 3 paragraphs above, and also with an explanation of the behavior that applications can expect from earlier kernels. > +Filesystems that wish to use this infrastructure should call > +mapping_set_error to record the error in the address_space when it > +occurs. The generic vfs code will then handle reporting the error when > +fsync is called, even if the fsync file operation returned 0. > + > +Filesystems are free to track errors internally if they choose (i.e. if > +they do keep track of how the pages were dirtied), but they should aim > +to provide the same (or better) error reporting semantics for when there > +are multiple writers. Those filesystems should avoid calling > +mapping_set_error in order to ensure that errors stored in the mapping > +aren't improperly reported by the generic filesystem code. > > struct address_space_operations > ------------------------------- > @@ -810,7 +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 > @@ -893,9 +928,10 @@ otherwise noted. > > release: called when the last reference to an open file is closed > > - fsync: called by the fsync(2) system call. Errors that were previously > + fsync: called by the fsync(2) system call. Errors that were previously > recorded using mapping_set_error will automatically be returned to > - the application and the file's error sequence advanced. > + the application and the struct file's error sequence advanced. > + See the section above on handling writeback errors. > > fasync: called by the fcntl(2) system call when asynchronous > (non-blocking) mode is enabled for a file
diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt index f201a77873f7..382190a872e5 100644 --- a/Documentation/filesystems/vfs.txt +++ b/Documentation/filesystems/vfs.txt @@ -576,12 +576,46 @@ 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. -If there is an error during writeback, then the address_space should be -marked with an error (typically using mapping_set_error), in order to -ensure that the error can later be reported to the application when an -fsync is issued. - -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, that error should 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 it 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. The generic vfs code will then handle reporting the error when +fsync is called, even if the fsync file operation returned 0. + +Filesystems are free to track errors internally if they choose (i.e. if +they do keep track of how the pages were dirtied), but they should aim +to provide the same (or better) error reporting semantics for when there +are multiple writers. Those filesystems should avoid calling +mapping_set_error in order to ensure that errors stored in the mapping +aren't improperly reported by the generic filesystem code. struct address_space_operations ------------------------------- @@ -810,7 +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 @@ -893,9 +928,10 @@ otherwise noted. release: called when the last reference to an open file is closed - fsync: called by the fsync(2) system call. Errors that were previously + fsync: called by the fsync(2) system call. Errors that were previously recorded using mapping_set_error will automatically be returned to - the application and the file's error sequence advanced. + the application and the struct file's error sequence advanced. + See the section above on handling writeback errors. fasync: called by the fcntl(2) system call when asynchronous (non-blocking) mode is enabled for a file
I waxed a little loquacious here, but I figured that more detail was better, and writeback error handling is so hard to get right. Cc: Jan Kara <jack@suse.cz> Signed-off-by: Jeff Layton <jlayton@redhat.com> --- Documentation/filesystems/vfs.txt | 54 ++++++++++++++++++++++++++++++++------- 1 file changed, 45 insertions(+), 9 deletions(-)