Re: [PATCH v3 10/20] fuse: set mapping error in writepage_locked when it fails

[Jeff Layton]

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.

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...

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.

> > 
> > > 
> > > > ---
> > > >  fs/fuse/file.c | 1 +
> > > >  1 file changed, 1 insertion(+)
> > > > 
> > > > diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> > > > index ec238fb5a584..07d0efcb050c 100644
> > > > --- a/fs/fuse/file.c
> > > > +++ b/fs/fuse/file.c
> > > > @@ -1669,6 +1669,7 @@ static int fuse_writepage_locked(struct page *page)
> > > >  err_free:
> > > >  	fuse_request_free(req);
> > > >  err:
> > > > +	mapping_set_error(page->mapping, error);
> > > >  	end_page_writeback(page);
> > > >  	return error;
> > > >  }
> > > > -- 
> > > > 2.9.3
> > > > 
> > > > 
> > 
> > -- 
> > Jeff Layton <jlayton@xxxxxxxxxx>

-- 
Jeff Layton <jlayton@xxxxxxxxxx>