On Wed, 2017-05-10 at 13:48 +0200, Jan Kara wrote: > On Tue 09-05-17 11:49:17, Jeff Layton wrote: > > Most filesystems currently use mapping_set_error and > > filemap_check_errors for setting and reporting/clearing writeback errors > > at the mapping level. filemap_check_errors is indirectly called from > > most of the filemap_fdatawait_* functions and from > > filemap_write_and_wait*. These functions are called from all sorts of > > contexts to wait on writeback to finish -- e.g. mostly in fsync, but > > also in truncate calls, getattr, etc. > > > > The non-fsync callers are problematic. We should be reporting writeback > > errors during fsync, but many places spread over the tree clear out > > errors before they can be properly reported, or report errors at > > nonsensical times. > > > > If I get -EIO on a stat() call, there is no reason for me to assume that > > it is because some previous writeback failed. The fact that it also > > clears out the error such that a subsequent fsync returns 0 is a bug, > > and a nasty one since that's potentially silent data corruption. > > > > This patch adds a small bit of new infrastructure for setting and > > reporting errors during address_space writeback. While the above was my > > original impetus for adding this, I think it's also the case that > > current fsync semantics are just problematic for userland. Most > > applications that call fsync do so to ensure that the data they wrote > > has hit the backing store. > > > > In the case where there are multiple writers to the file at the same > > time, this is really hard to determine. The first one to call fsync will > > see any stored error, and the rest get back 0. The processes with open > > fds may not be associated with one another in any way. They could even > > be in different containers, so ensuring coordination between all fsync > > callers is not really an option. > > > > One way to remedy this would be to track what file descriptor was used > > to dirty the file, but that's rather cumbersome and would likely be > > slow. However, there is a simpler way to improve the semantics here > > without incurring too much overhead. > > > > This set adds an errseq_t to struct address_space, and a corresponding > > one is added to struct file. Writeback errors are recorded in the > > mapping's errseq_t, and the one in struct file is used as the "since" > > value. > > > > This changes the semantics of the Linux fsync implementation such that > > applications can now use it to determine whether there were any > > writeback errors since fsync(fd) was last called (or since the file was > > opened in the case of fsync having never been called). > > > > Note that those writeback errors may have occurred when writing data > > that was dirtied via an entirely different fd, but that's the case now > > with the current mapping_set_error/filemap_check_error infrastructure. > > This will at least prevent you from getting a false report of success. > > > > The new behavior is still consistent with the POSIX spec, and is more > > reliable for application developers. This patch just adds some basic > > infrastructure for doing this. Later patches will change the existing > > code to use this new infrastructure. > > > > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx> > > Just one nit below. Otherwise the patch looks good to me. You can add: > > Reviewed-by: Jan Kara <jack@xxxxxxx> > > > diff --git a/fs/file_table.c b/fs/file_table.c > > index 954d510b765a..d6138b6411ff 100644 > > --- a/fs/file_table.c > > +++ b/fs/file_table.c > > @@ -168,6 +168,7 @@ struct file *alloc_file(const struct path *path, fmode_t mode, > > file->f_path = *path; > > file->f_inode = path->dentry->d_inode; > > file->f_mapping = path->dentry->d_inode->i_mapping; > > + file->f_wb_err = filemap_sample_wb_error(file->f_mapping); > > Why do you sample here when you also sample in do_dentry_open()? I didn't > find any alloc_file() callers that would possibly care about writeback > errors... > > Honza I basically used the setting of f_mapping as a guideline as to where to sample it for initialization. My thinking was that if f_mapping ever ended up different then you'd probably also want f_wb_err to be resampled anyway. I can drop this hunk if you think we don't need it. -- Jeff Layton <jlayton@xxxxxxxxxx>
