linux-kernel - Re: [PATCH v3 20/20] gfs2: clean up some filemap

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Date:   Mon, 24 Apr 2017 13:52:36 -0400
From:   Jeff Layton <jlayton@...hat.com>
To:     Bob Peterson <rpeterso@...hat.com>
Cc:     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 <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 20/20] gfs2: clean up some filemap_* calls

On Mon, 2017-04-24 at 13:41 -0400, Bob Peterson wrote:
> Hi,
> 
> ----- Original Message -----
> > On Mon, 2017-04-24 at 10:12 -0400, Bob Peterson wrote:
> > > > +	filemap_write_and_wait_range(mapping, gl->gl_vm.start, gl->gl_vm.end);
> > > 
> > > This should probably have "error = ", no?
> > > 
> > 
> > This error is discarded in the current code after resetting the error in
> > the mapping. With the earlier patches in this set we don't need to reset
> > the error like this anymore.
> > 
> > Now, if this code should doing something else with those errors, then
> > that's a separate problem.
> 
> Okay, I see. My bad.
>  
> > > >  	gfs2_ail_empty_gl(gl);
> > > >  
> > > >  	spin_lock(&gl->gl_lockref.lock);
> > > > @@ -225,12 +223,10 @@ static void inode_go_sync(struct gfs2_glock *gl)
> > > >  	filemap_fdatawrite(metamapping);
> > > >  	if (ip) {
> > > >  		struct address_space *mapping = ip->i_inode.i_mapping;
> > > > -		filemap_fdatawrite(mapping);
> > > > -		error = filemap_fdatawait(mapping);
> > > > -		mapping_set_error(mapping, error);
> > > > +		filemap_write_and_wait(mapping);
> > > > +	} else {
> > > > +		filemap_fdatawait(metamapping);
> > > >  	}
> > > > -	error = filemap_fdatawait(metamapping);
> > > > -	mapping_set_error(metamapping, error);
> > > 
> > > This part doesn't look right at all. There's a big difference in gfs2
> > > between
> > > mapping and metamapping. We need to wait for metamapping regardless.
> > > 
> > 
> > ...and this should wait. Basically, filemap_write_and_wait does
> > filemap_fdatawrite and then filemap_fdatawait. This is mostly just
> > replacing the existing code with a more concise helper.
> 
> But this isn't a simple replacement with a helper. This is two different
> address spaces (mapping and metamapping) and you added an else in there.
> 
> So with this patch metamapping gets written, and if there's an ip,
> mapping gets written but it doesn't wait for metamapping. Unless
> I'm missing something.
> 
> You could replace both filemap_fdatawrites with the helper instead.
> Today's code is structured as:
> 
> (a) write metamapping
> if (ip)
>     (b) write mapping
>     (c) wait for mapping
> (d) wait for metamapping
> 
> If you use the helper for both, it becomes, (a & d)(b & c) which is probably
> acceptable. (I think we just tried to optimize what the elevator was doing).
> 
> But the way you've got it coded here still looks wrong. It looks like:
> (a)
> if (ip)
>    (b & c)
> ELSE -
>    (d)
> 
> So (d) (metamapping) isn't guaranteed to be synced at the end of the function.
> Of course, you know the modified helper functions better than I do.
> What am I missing?
> 
> 

<facepalm>
You're right of course. I'll fix that up in my tree.

Thanks!
-- 
Jeff Layton <jlayton@...hat.com>