Re: [RFC PATCH 1/4] fs: new infrastructure for writeback error handling and reporting

[Jeff Layton <jlayton@redhat.com>]

On Mon, 2017-04-03 at 10:12 +0300, Nikolay Borisov wrote:
> 
> On 31.03.2017 22:26, 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.
> > 
> > It's those non-fsync callers that are problematic. We should be
> > reporting writeback errors during fsync, but many places in the code
> > clear out errors before they can be properly reported, or report errors
> > at nonsensical times. If I get -EIO on a stat() call, how do I know that
> > was because writeback failed?
> > 
> > This patch adds a small bit of new infrastructure for setting and
> > reporting errors during pagecache 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
> > fd 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 a wb_error field and a sequence counter to the
> > address_space, and a corresponding sequence counter in the struct file.
> > When errors are reported during writeback, we set the error field in the
> > mapping and increment the sequence counter.
> > 
> > When fsync or flush is called, we check the sequence in the file vs. the
> > one in the mapping. If the file's counter is behind the one in the
> > mapping, then we update the sequence counter in the file to the value of
> > the one in the mapping and report the error. If the file is "caught up"
> > then we just report 0.
> > 
> > This changes the semantics of fsync 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 basic idea here is for filesystems to use filemap_set_wb_error to
> > set the error in the mapping when there are writeback errors, and then
> > have the fsync and flush operations call filemap_report_wb_error just
> > before returning to ensure that those errors get reported properly.
> > 
> > Eventually, it may make sense to move the reporting into the generic
> > vfs_fsync_range helper, but doing it this way for now makes it simpler
> > to convert filesystems to the new API individually.
> 
> There is already a mapping_set_error API which sets flags in
> mapping->flags (AS_EIO/AS_ENOSPC). Aren't you essentially duplicating
> some of the semantics of that API ?

Yes, more or less for now. The arguments of mapping_set_error and
filemap_set_wb_error are the same, but they do different things with the
error.

The plan is eventually to eliminate mapping_set_error and convert
everything over to use the new infrastructure I'm adding. That's
difficult to do all at once however, so for now some duplication is
necessary.
-- 
Jeff Layton <jlayton@redhat.com>