linux-ext4 - Re: [PATCH v3 10/20] fuse: set mapping error in writepage

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Date:   Tue, 25 Apr 2017 10:17:20 +0200
From:   Jan Kara <jack@...e.cz>
To:     Jeff Layton <jlayton@...hat.com>
Cc:     Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-cifs@...r.kernel.org,
        linux-mm@...ck.org, jfs-discussion@...ts.sourceforge.net,
        linux-xfs@...r.kernel.org, cluster-devel@...hat.com,
        linux-f2fs-devel@...ts.sourceforge.net,
        v9fs-developer@...ts.sourceforge.net, osd-dev@...n-osd.org,
        linux-nilfs@...r.kernel.org, linux-block@...r.kernel.org,
        dhowells@...hat.com, akpm@...ux-foundation.org, hch@...radead.org,
        ross.zwisler@...ux.intel.com, mawilcox@...rosoft.com,
        jack@...e.com, viro@...iv.linux.org.uk, corbet@....net,
        neilb@...e.de, clm@...com, tytso@....edu, axboe@...nel.dk
Subject: Re: [PATCH v3 10/20] fuse: set mapping error in writepage_locked
 when it fails

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@...hat.com>
> > 
> > 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.

								Honza

> 
> > 
> > > ---
> > >  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@...hat.com>
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR