On Tue, 2017-04-25 at 13:19 +0200, Jan Kara wrote: > On Tue 25-04-17 06:35:13, Jeff Layton wrote: > > On Tue, 2017-04-25 at 10:17 +0200, Jan Kara wrote: > > > On Mon 24-04-17 13:14:36, Jeff Layton wrote: > > > > On Mon, 2017-04-24 at 18:04 +0200, Jan Kara wrote: > > > > > On Mon 24-04-17 09:22:49, Jeff Layton wrote: > > > > > > This ensures that we see errors on fsync when writeback fails. > > > > > > > > > > > > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx> > > > > > > > > > > Hum, but do we really want to clobber mapping errors with temporary stuff > > > > > like ENOMEM? Or do you want to handle that in mapping_set_error? > > > > > > > > > > > > > Right now we don't really have such a thing as temporary errors in the > > > > writeback codepath. If you return an error here, the data doesn't stay > > > > dirty or anything, and I think we want to ensure that that gets reported > > > > via fsync. > > > > > > > > I'd like to see us add better handling for retryable errors for stuff > > > > like ENOMEM or EAGAIN. I think this is the first step toward that > > > > though. Once we have more consistent handling of writeback errors in > > > > general, then we can start doing more interesting things with retryable > > > > errors. > > > > > > > > So yeah, I this is the right thing to do for now. > > > > > > OK, fair enough. And question number 2): > > > > > > Who is actually responsible for setting the error in the mapping when error > > > happens inside ->writepage()? Is it the ->writepage() callback or the > > > caller of ->writepage()? Or something else? Currently it seems to be a > > > strange mix (e.g. mm/page-writeback.c: __writepage() calls > > > mapping_set_error() when ->writepage() returns error) so I'd like to > > > understand what's the plan and have that recorded in the changelogs. > > > > > > > That's an excellent question. > > > > I think we probably want the writepage/launder_page operations to call > > mapping_set_error. That makes it possible for filesystems (e.g. NFS) to > > handle their own error tracking and reporting without using the new > > infrastructure. If they never call mapping_set_error then we'll always > > just return whatever their ->fsync operation returns on an fsync. > > OK, makes sense. It is also in line with what you did for DAX, 9p, or here > for FUSE. So feel free to add: > > Reviewed-by: Jan Kara <jack@xxxxxxx> > > for this patch but please also add a sentense that ->writepage() is > responsible for calling mapping_set_error() if it fails and page is not > redirtied to the changelogs of patches changing writepage handlers. > > > I'll make another pass through the tree and see whether we have some > > mapping_set_error calls that should be removed, and will flesh out > > vfs.txt to state this. Maybe that file needs a whole section on > > writeback error reporting? Hmmm... > > I think it would be nice to have all the logic described in one place. So > +1 from me. > > > That probably also means that I should drop patch 8 from this series > > (mm: ensure that we set mapping error if writeout fails), since that > > should be happening in writepage already. > > Yes. > > Honza (cc'ing Jon since I'm proposing a doc update) Here's what I'm thinking for a vfs.txt update after this series. The section on writeback_control could probably be more specific. ----------------------8<------------------------- diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt index 94dd27ef4a76..aa912b65792a 100644 --- a/Documentation/filesystems/vfs.txt +++ b/Documentation/filesystems/vfs.txt @@ -576,7 +576,23 @@ 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 tells the writepage and writepages operations something +about the nature of and reason for the writeback request, and the +constraints under which it is being done. It is also used to track state +between successive writeback requests. + +When there is an error during writeback, then an error should be +reported to fsync on all file descriptors that were open at the time of +the error. This is typically done by setting the wb_err value in the +address_space via mapping_set_error when writeback errors occur. The +vfs-layer fsync code will then report the errors on a per-fd basis. + +Filesystems are free to track errors internally if they choose, but they +should aim to provide the same semantics for error reporting when there +are multiple writers. Filesystems that track their own errors 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 ------------------------------- @@ -888,7 +904,9 @@ 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. Errors that were previously + recorded using mapping_set_error will automatically be returned to + the application and the file's error sequence advanced. fasync: called by the fcntl(2) system call when asynchronous (non-blocking) mode is enabled for a file
