Subject: Re: why are WB_SYNC_NONE COMMITs being done with FLUSH_SYNC set ?
From: Trond Myklebust <trond.myklebust@fys.uio.no>
To: Christoph Hellwig <hch@infradead.org>
Cc: Jeff Layton <jlayton@redhat.com>, fengguang.wu@gmail.com,
        linux-nfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org
In-Reply-To: <20100819143710.GA4752@infradead.org>
References: <20100819101525.076831ad@barsoom.rdu.redhat.com>
	 <20100819143710.GA4752@infradead.org>
Content-Type: text/plain; charset="UTF-8"
Date: Thu, 19 Aug 2010 10:58:25 -0400
Message-ID: <1282229905.6199.19.camel@heimdal.trondhjem.org>
Sender: linux-nfs-owner@vger.kernel.org
MIME-Version: 1.0

On Thu, 2010-08-19 at 10:37 -0400, Christoph Hellwig wrote:
> On Thu, Aug 19, 2010 at 10:15:25AM -0400, Jeff Layton wrote:
> > I'm looking at backporting some upstream changes to earlier kernels,
> > and ran across something I don't quite understand...
> > 
> > In nfs_commit_unstable_pages, we set the flags to FLUSH_SYNC. We then
> > zero out the flags if wbc->nonblocking or wbc->for_background is set.
> > 
> > Shouldn't we also clear it out if wbc->sync_mode == WB_SYNC_NONE ?
> > WB_SYNC_NONE means "don't wait on anything", so shouldn't that include
> > not waiting on the COMMIT to complete?
> 
> I've been trying to figure out what the nonblocking flag is supposed
> to mean for a while now.
> 
> It basically disappeared in commit 0d99519efef15fd0cf84a849492c7b1deee1e4b7
> 
> 	"writeback: remove unused nonblocking and congestion checks"
> 
> from Wu.  What's left these days is a couple of places in local copies
> of write_cache_pages (afs, cifs), and a couple of checks in random
> writepages instances (afs, block_write_full_page, ceph, nfs, reiserfs, xfs)
> and the use in nfs_write_inode.  It's only actually set for memory
> migration and pageout, that is VM writeback.
> 
> To me it really doesn't make much sense, but maybe someone has a better
> idea what it is for.
> 
> > +	if (wbc->nonblocking || wbc->for_background ||
> > +	    wbc->sync_mode == WB_SYNC_NONE)
> 
> You could remove the nonblocking and for_background checks as
> these impliy WB_SYNC_NONE.

To me that sounds fine. I've also been trying to wrap my head around the
differences between 'nonblocking', 'for_background', 'for_reclaim' and
'for_kupdate' and how the filesystem is supposed to treat them.

Aside from the above, I've used 'for_reclaim', 'for_kupdate' and
'for_background' in order to adjust the RPC request's queuing priority
(high in the case of 'for_reclaim' and low for the other two).

Cheers
  Trond