Re: why are WB_SYNC_NONE COMMITs being done with FLUSH_SYNC set ?

[Sage Weil <sage@newdream.net>]

On Fri, 20 Aug 2010, Wu Fengguang wrote:

> [add CC to afs/cifs/ceph maintainers]
> 
> On Fri, Aug 20, 2010 at 07:55:53AM +0800, Wu Fengguang wrote:
> > On Thu, Aug 19, 2010 at 10:37:10AM -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.
> > 
> > In principle all nonblocking checks in ->writepages should be removed.
> > 
> > (My original patch does have chunks for afs/cifs that somehow get
> >  dropped in the process, and missed ceph because it's not upstream
> >  when I started patch..)

I'll queue up a fix for Ceph's ->writepages in my tree.

Thanks!
sage


> > 
> > > 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.
> >  
> > Since migration and pageout still set nonblocking for ->writepage, we
> > may keep them in the near future, until VM does not start IO on itself.
> > 
> > > > +	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.
> > 
> > Agreed.
> > 
> > Thanks,
> > Fengguang
> 

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