Re: [PATCH 5/6] NFSD: leverage DIO alignment to selectively issue O_DIRECT reads and writes

[Jeff Layton <jlayton@kernel.org>]

On Wed, 2025-06-11 at 10:42 -0400, Chuck Lever wrote:
> On 6/10/25 4:57 PM, Mike Snitzer wrote:
> > IO must be aligned, otherwise it falls back to using buffered IO.
> > 
> > RWF_DONTCACHE is _not_ currently used for misaligned IO (even when
> > nfsd/enable-dontcache=1) because it works against us (due to RMW
> > needing to read without benefit of cache), whereas buffered IO enables
> > misaligned IO to be more performant.
> > 
> > Signed-off-by: Mike Snitzer <snitzer@kernel.org>
> > ---
> >  fs/nfsd/vfs.c | 40 ++++++++++++++++++++++++++++++++++++----
> >  1 file changed, 36 insertions(+), 4 deletions(-)
> > 
> > diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> > index e7cc8c6dfbad..a942609e3ab9 100644
> > --- a/fs/nfsd/vfs.c
> > +++ b/fs/nfsd/vfs.c
> > @@ -1064,6 +1064,22 @@ __be32 nfsd_splice_read(struct svc_rqst *rqstp, struct svc_fh *fhp,
> >  	return nfsd_finish_read(rqstp, fhp, file, offset, count, eof, host_err);
> >  }
> >  
> > +static bool is_dio_aligned(const struct iov_iter *iter, loff_t offset,
> > +			   const u32 blocksize)
> > +{
> > +	u32 blocksize_mask;
> > +
> > +	if (!blocksize)
> > +		return false;
> > +
> > +	blocksize_mask = blocksize - 1;
> > +	if ((offset & blocksize_mask) ||
> > +	    (iov_iter_alignment(iter) & blocksize_mask))
> > +		return false;
> > +
> > +	return true;
> > +}
> > +
> >  /**
> >   * nfsd_iter_read - Perform a VFS read using an iterator
> >   * @rqstp: RPC transaction context
> > @@ -1107,8 +1123,16 @@ __be32 nfsd_iter_read(struct svc_rqst *rqstp, struct svc_fh *fhp,
> >  	trace_nfsd_read_vector(rqstp, fhp, offset, *count);
> >  	iov_iter_bvec(&iter, ITER_DEST, rqstp->rq_bvec, v, *count);
> >  
> > -	if (nfsd_enable_dontcache)
> > -		flags |= RWF_DONTCACHE;
> > +	if (nfsd_enable_dontcache) {
> > +		if (is_dio_aligned(&iter, offset, nf->nf_dio_read_offset_align))
> > +			flags |= RWF_DIRECT;
> > +		/* FIXME: not using RWF_DONTCACHE for misaligned IO because it works
> > +		 * against us (due to RMW needing to read without benefit of cache),
> > +		 * whereas buffered IO enables misaligned IO to be more performant.
> > +		 */
> > +		//else
> > +		//	flags |= RWF_DONTCACHE;
> > +	}
> >  
> >  	host_err = vfs_iter_read(file, &iter, &ppos, flags);
> >  	return nfsd_finish_read(rqstp, fhp, file, offset, count, eof, host_err);
> > @@ -1217,8 +1241,16 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp,
> >  	nvecs = xdr_buf_to_bvec(rqstp->rq_bvec, rqstp->rq_maxpages, payload);
> >  	iov_iter_bvec(&iter, ITER_SOURCE, rqstp->rq_bvec, nvecs, *cnt);
> >  
> > -	if (nfsd_enable_dontcache)
> > -		flags |= RWF_DONTCACHE;
> > +	if (nfsd_enable_dontcache) {
> > +		if (is_dio_aligned(&iter, offset, nf->nf_dio_offset_align))
> > +			flags |= RWF_DIRECT;
> > +		/* FIXME: not using RWF_DONTCACHE for misaligned IO because it works
> > +		 * against us (due to RMW needing to read without benefit of cache),
> > +		 * whereas buffered IO enables misaligned IO to be more performant.
> > +		 */
> > +		//else
> > +		//	flags |= RWF_DONTCACHE;
> > +	}
> 
> IMO adding RWF_DONTCACHE first then replacing it later in the series
> with a form of O_DIRECT is confusing. Also, why add RWF_DONTCACHE here
> and then take it away "because it doesn't work"?
> 
> But OK, your series is really a proof-of-concept. Something to work out
> before it is merge-ready, I guess.
> 
> It is much more likely for NFS READ requests to be properly aligned.
> Clients are generally good about that. NFS WRITE request alignment
> is going to be arbitrary. Fwiw.
> 
> However, one thing we discussed at bake-a-thon was what to do about
> unstable WRITEs. For unstable WRITEs, the server has to cache the
> write data at least until the client sends a COMMIT. Otherwise the
> server will have to convert all UNSTABLE writes to FILE_SYNC writes,
> and that can have performance implications.
> 

If we're doing synchronous, direct I/O writes then why not just respond
with FILE_SYNC? The write should be on the platter by the time it
returns.

> One thing you might consider is to continue using the page cache for
> unstable WRITEs, and then use fadvise DONTNEED after a successful
> COMMIT operation to reduce page cache footprint. Unstable writes to
> the same range of the file might be a problem, however.

Since the client sends almost everything UNSTABLE, that would probably
erase most of the performance win. The only reason I can see to use
buffered I/O in this mode would be because we had to deal with an
unaligned write and need to do a RMW cycle on a block.

The big question is whether mixing buffered and direct I/O writes like
this is safe across all exportable filesystems. I'm not yet convinced
of that.
-- 
Jeff Layton <jlayton@kernel.org>