Subject: Re: [PATCH] NFS: filelayout should use nfs_generic_pg_test
From: Trond Myklebust <Trond.Myklebust@netapp.com>
To: Benny Halevy <bhalevy@panasas.com>
Cc: Weston Andros Adamson <dros@netapp.com>,
        Boaz Harrosh <bharrosh@panasas.com>, trond@netapp.com,
        linux-nfs@vger.kernel.org
Date: Wed, 01 Jun 2011 15:29:51 -0400
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On Wed, 2011-06-01 at 22:13 +0300, Benny Halevy wrote: 
> On 2011-06-01 21:07, Trond Myklebust wrote:
> > On Wed, 2011-06-01 at 17:51 +0300, Benny Halevy wrote: 
> >> I think the following should work:
> >>
> >> Benny
> >>
> >> git diff --stat -p -M
> >>  fs/nfs/nfs4filelayout.c |   10 ++++++++++
> >>  1 files changed, 10 insertions(+), 0 deletions(-)
> >>
> >> diff --git a/fs/nfs/nfs4filelayout.c b/fs/nfs/nfs4filelayout.c
> >> index 4269088..9f1d445 100644
> >> --- a/fs/nfs/nfs4filelayout.c
> >> +++ b/fs/nfs/nfs4filelayout.c
> >> @@ -661,6 +661,16 @@ filelayout_pg_test(struct nfs_pageio_descriptor
> >> *pgio, struct nfs_page *prev,
> >>  	u64 p_stripe, r_stripe;
> >>  	u32 stripe_unit;
> >>
> >> +	/*
> >> +	 * FIXME: ideally we should be able to coalesce all requests
> >> +	 * that are not block boundary aligned, but currently this
> >> +	 * is problematic for the case of bsize < PAGE_CACHE_SIZE,
> >> +	 * since nfs_flush_multi and nfs_pagein_multi assume you
> >> +	 * can have only one struct nfs_page.
> >> +	 */
> >> +	if (desc->pg_bsize < PAGE_SIZE)
> >> +		return 0;
> >> +
> >>  	if (!pnfs_generic_pg_test(pgio, prev, req))
> >>  		return 0;
> > 
> > So, there are several things that bother me about pnfs_generic_pg_test()
> > too now that I'm looking more closely at it:
> > 
> >      1. If the intention is to coalesce 'prev' and 'req', shouldn't we
> >         be asking for a layout with req_offset(prev) instead of
> >         req_offset(req)? 
> >      2. If we're only requesting a layout of length pg_count, don't we
> >         still need to test the layout length that the server actually
> >         returned before we can allow the coalescing? 
> >      3. if (!pgio->lseg), shouldn't we be returning an error of some
> >         sort? Right now we're returning 'true', and allowing the
> >         coalesce to occur. 
> >      4. Furthermore, shouldn't that test guarding the
> >         pnfs_update_layout() call rather be an 'if (pgio->pg_lseg ==
> >         NULL)' instead of looking at the values of pg_count and
> >         prev->wb_bytes?
> > 
> 
> or rather we get the layout for the first page in
> nfs_pageio_do_add_request when desc->pg_count == 0?

I can live with a desc->pg_init() callback or rather, converting
pg_test() and pg_doio() into a

struct nfs_pageio_ops {
	int (*pg_init)(struct nfs_pageio_descriptor *desc, struct nfs_page *req);
	bool (*pg_test)(struct nfs_pageio_descriptor *desc, struct nfs_page *prev, struct nfs_page *req);
	int (*pg_doio)(struct nfs_pageio_descriptor *desc);
};

and then replacing the two callback functions in the existing struct
nfs_pageio_descriptor with a single pointer to a 'const struct
nfs_pageio_ops'...

> Then, this lseg would be useful for nfs_flush_multi
> if we failed to coalesce, or we failed to get a layout
> altogether we go the nfs path and can reset pg_test to
> nfs_generic_pg_test.

It would presumably also get rid of all those pnfs_update_layout() calls
in read.c and write.c.

-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@netapp.com
www.netapp.com