Re: [PATCH v4 15/27] fs: retrofit old error reporting API onto new infrastructure

[Jan Kara]

On Fri 19-05-17 15:20:52, Jeff Layton wrote:
> On Mon, 2017-05-15 at 12:42 +0200, Jan Kara wrote:
> > On Tue 09-05-17 11:49:18, Jeff Layton wrote:
> > > Now that we have a better way to store and report errors that occur
> > > during writeback, we need to convert the existing codebase to use it. We
> > > could just adapt all of the filesystem code and related infrastructure
> > > to the new API, but that's a lot of churn.
> > > 
> > > When it comes to setting errors in the mapping, filemap_set_wb_error is
> > > a drop-in replacement for mapping_set_error. Turn that function into a
> > > simple wrapper around the new one.
> > > 
> > > Because we want to ensure that writeback errors are always reported at
> > > fsync time, inject filemap_report_wb_error calls much closer to the
> > > syscall boundary, in call_fsync.
> > > 
> > > For fsync calls (and things like the nfsd equivalent), we either return
> > > the error that the fsync operation returns, or the one returned by
> > > filemap_report_wb_error. In both cases, we advance the file->f_wb_err to
> > > the latest value. This allows us to provide new fsync semantics that
> > > will return errors that may have occurred previously and been viewed
> > > via other file descriptors.
> > > 
> > > The final piece of the puzzle is what to do about filemap_check_errors
> > > calls that are being called directly or via filemap_* functions. Here,
> > > we must take a little "creative license".
> > > 
> > > Since we now handle advancing the file->f_wb_err value at the generic
> > > filesystem layer, we no longer need those callers to clear errors out
> > > of the mapping or advance an errseq_t.
> > > 
> > > A lot of the existing codebase relies on being getting an error back
> > > from those functions when there is a writeback problem, so we do still
> > > want to have them report writeback errors somehow.
> > > 
> > > When reporting writeback errors, we will always report errors that have
> > > occurred since a particular point in time. With the old writeback error
> > > reporting, the time we used was "since it was last tested/cleared" which
> > > is entirely arbitrary and potentially racy. Now, we can at least report
> > > the latest error that has occurred since an arbitrary point in time
> > > (represented as a sampled errseq_t value).
> > > 
> > > In the case where we don't have a struct file to work with, this patch
> > > just has the wrappers sample the current mapping->wb_err value, and use
> > > that as an arbitrary point from which to check for errors.
> > 
> > I think this is really dangerous and we shouldn't do this. You are quite
> > likely to lose IO errors in such calls because you will ignore all errors
> > that happened during previous background writeback or even for IO that
> > managed to complete before we called filemap_fdatawait(). Maybe we need to
> > keep the original set-clear-bit IO error reporting for these cases, until
> > we can convert them to fdatawait_range_since()?
> > 
> > > That's probably not "correct" in all cases, particularly in the case of
> > > something like filemap_fdatawait, but I'm not sure it's any worse than
> > > what we already have, and this gives us a basis from which to work.
> > > 
> > > A lot of those callers will likely want to change to a model where they
> > > sample the errseq_t much earlier (perhaps when starting a transaction),
> > > store it in an appropriate place and then use that value later when
> > > checking to see if an error occurred.
> > > 
> > > That will almost certainly take some involvement from other subsystem
> > > maintainers. I'm quite open to adding new API functions to help enable
> > > this if that would be helpful, but I don't really want to do that until
> > > I better understand what's needed.
> > > 
> > > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> > 
> > ...
> > 
> > > diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
> > > index 5f7317875a67..7ce13281925f 100644
> > > --- a/fs/f2fs/file.c
> > > +++ b/fs/f2fs/file.c
> > > @@ -187,6 +187,7 @@ static int f2fs_do_sync_file(struct file *file, loff_t start, loff_t end,
> > >  		.nr_to_write = LONG_MAX,
> > >  		.for_reclaim = 0,
> > >  	};
> > > +	errseq_t since = READ_ONCE(file->f_wb_err);
> > >  
> > >  	if (unlikely(f2fs_readonly(inode->i_sb)))
> > >  		return 0;
> > > @@ -265,6 +266,8 @@ static int f2fs_do_sync_file(struct file *file, loff_t start, loff_t end,
> > >  	}
> > >  
> > >  	ret = wait_on_node_pages_writeback(sbi, ino);
> > > +	if (ret == 0)
> > > +		ret = filemap_check_wb_error(NODE_MAPPING(sbi), since);
> > >  	if (ret)
> > >  		goto out;
> > 
> > So this conversion looks wrong and actually points to a larger issue with
> > the scheme. The problem is there are two mappings that come into play here
> > - file_inode(file)->i_mapping which is the data mapping and
> > NODE_MAPPING(sbi) which is the metadata mapping (and this is not a problem
> > specific to f2fs. For example ext2 also uses this scheme where block
> > devices' mapping is the metadata mapping). And we need to merge error
> > information from these two mappings so for the stamping scheme to work,
> > we'd need two stamps stored in struct file. One for data mapping and one
> > for metadata mapping. Or maybe there's some more clever scheme but for now
> > I don't see one...
> > 
> > 								Honza
> 
> In the case of something like ext2, could we instead get away with just
> marking the data mapping of the inode with an error if the metadata
> writeout fails?
> 
> Then we could just have write_inode operations call mapping_set_error on
> inode->i_mapping when they're going to return an error. That should be
> functionally equivalent, I'd think.
> 
> The catch there is that that requires a 1:1 data:metadata mapping, and
> I'm not sure that that is the case (or will always be, even if it is
> now).

So for ext2 / ext4 in nojournal mode this should work - we track all
relevant metadata in mapping->private_list. But I cannot really comment
on other filesystems like f2fs...

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR