Re: [PATCH v1 3/3] NFSD: Implement NFSD_IO_DIRECT for NFS READ

[Chuck Lever <cel@kernel.org>]

On 9/9/25 7:37 PM, NeilBrown wrote:
> On Wed, 10 Sep 2025, Chuck Lever wrote:
>> From: Chuck Lever <chuck.lever@oracle.com>
>>
>> Add an experimental option that forces NFS READ operations to use
>> direct I/O instead of reading through the NFS server's page cache.
>>
>> There are already other layers of caching:
>>  - The page cache on NFS clients
>>  - The block device underlying the exported file system
>>
>> The server's page cache, in many cases, is unlikely to provide
>> additional benefit. Some benchmarks have demonstrated that the
>> server's page cache is actively detrimental for workloads whose
>> working set is larger than the server's available physical memory.
>>
>> For instance, on small NFS servers, cached NFS file content can
>> squeeze out local memory consumers. For large sequential workloads,
>> an enormous amount of data flows into and out of the page cache
>> and is consumed by NFS clients exactly once -- caching that data
>> is expensive to do and totally valueless.
>>
>> For now this is a hidden option that can be enabled on test
>> systems for benchmarking. In the longer term, this option might
>> be enabled persistently or per-export. When the exported file
>> system does not support direct I/O, NFSD falls back to using
>> either DONTCACHE or buffered I/O to fulfill NFS READ requests.
>>
>> Suggested-by: Mike Snitzer <snitzer@kernel.org>
>> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
>> ---
>>  fs/nfsd/debugfs.c |  2 ++
>>  fs/nfsd/nfsd.h    |  1 +
>>  fs/nfsd/trace.h   |  1 +
>>  fs/nfsd/vfs.c     | 78 +++++++++++++++++++++++++++++++++++++++++++++++
>>  4 files changed, 82 insertions(+)
>>
>> diff --git a/fs/nfsd/debugfs.c b/fs/nfsd/debugfs.c
>> index ed2b9e066206..00eb1ecef6ac 100644
>> --- a/fs/nfsd/debugfs.c
>> +++ b/fs/nfsd/debugfs.c
>> @@ -44,6 +44,7 @@ DEFINE_DEBUGFS_ATTRIBUTE(nfsd_dsr_fops, nfsd_dsr_get, nfsd_dsr_set, "%llu\n");
>>   * Contents:
>>   *   %0: NFS READ will use buffered IO
>>   *   %1: NFS READ will use dontcache (buffered IO w/ dropbehind)
>> + *   %2: NFS READ will use direct IO
>>   *
>>   * This setting takes immediate effect for all NFS versions,
>>   * all exports, and in all NFSD net namespaces.
>> @@ -64,6 +65,7 @@ static int nfsd_io_cache_read_set(void *data, u64 val)
>>  		nfsd_io_cache_read = NFSD_IO_BUFFERED;
>>  		break;
>>  	case NFSD_IO_DONTCACHE:
>> +	case NFSD_IO_DIRECT:
>>  		/*
>>  		 * Must disable splice_read when enabling
>>  		 * NFSD_IO_DONTCACHE.
>> diff --git a/fs/nfsd/nfsd.h b/fs/nfsd/nfsd.h
>> index ea87b42894dd..bdb60ee1f1a4 100644
>> --- a/fs/nfsd/nfsd.h
>> +++ b/fs/nfsd/nfsd.h
>> @@ -157,6 +157,7 @@ enum {
>>  	/* Any new NFSD_IO enum value must be added at the end */
>>  	NFSD_IO_BUFFERED,
>>  	NFSD_IO_DONTCACHE,
>> +	NFSD_IO_DIRECT,
>>  };
>>  
>>  extern u64 nfsd_io_cache_read __read_mostly;
>> diff --git a/fs/nfsd/trace.h b/fs/nfsd/trace.h
>> index e5af0d058fd0..88901df5fbb1 100644
>> --- a/fs/nfsd/trace.h
>> +++ b/fs/nfsd/trace.h
>> @@ -464,6 +464,7 @@ DEFINE_EVENT(nfsd_io_class, nfsd_##name,	\
>>  DEFINE_NFSD_IO_EVENT(read_start);
>>  DEFINE_NFSD_IO_EVENT(read_splice);
>>  DEFINE_NFSD_IO_EVENT(read_vector);
>> +DEFINE_NFSD_IO_EVENT(read_direct);
>>  DEFINE_NFSD_IO_EVENT(read_io_done);
>>  DEFINE_NFSD_IO_EVENT(read_done);
>>  DEFINE_NFSD_IO_EVENT(write_start);
>> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
>> index 441267d877f9..9665454743eb 100644
>> --- a/fs/nfsd/vfs.c
>> +++ b/fs/nfsd/vfs.c
>> @@ -1074,6 +1074,79 @@ __be32 nfsd_splice_read(struct svc_rqst *rqstp, struct svc_fh *fhp,
>>  	return nfsd_finish_read(rqstp, fhp, file, offset, count, eof, host_err);
>>  }
>>  
>> +/*
>> + * The byte range of the client's READ request is expanded on both
>> + * ends until it meets the underlying file system's direct I/O
>> + * alignment requirements. After the internal read is complete, the
>> + * byte range of the NFS READ payload is reduced to the byte range
>> + * that was originally requested.
>> + *
>> + * Note that a direct read can be done only when the xdr_buf
>> + * containing the NFS READ reply does not already have contents in
>> + * its .pages array. This is due to potentially restrictive
>> + * alignment requirements on the read buffer. When .page_len and
>> + * @base are zero, the .pages array is guaranteed to be page-
>> + * aligned.
> 
> Where do we test that this condition is met?
> 
> I can see that nfsd_direct_read() is only called if "base" is zero, but
> that means the current contents of the .pages array are page-aligned,
> not that there are now.
> 
>> + */
>> +static noinline_for_stack __be32
>> +nfsd_direct_read(struct svc_rqst *rqstp, struct svc_fh *fhp,
>> +		 struct nfsd_file *nf, loff_t offset, unsigned long *count,
>> +		 u32 *eof)
>> +{
>> +	loff_t dio_start, dio_end;
>> +	unsigned long v, total;
>> +	struct iov_iter iter;
>> +	struct kiocb kiocb;
>> +	ssize_t host_err;
>> +	size_t len;
>> +
>> +	init_sync_kiocb(&kiocb, nf->nf_file);
>> +	kiocb.ki_flags |= IOCB_DIRECT;
>> +
>> +	/* Read a properly-aligned region of bytes into rq_bvec */
>> +	dio_start = round_down(offset, nf->nf_dio_read_offset_align);
>> +	dio_end = round_up(offset + *count, nf->nf_dio_read_offset_align);
>> +
>> +	kiocb.ki_pos = dio_start;
>> +
>> +	v = 0;
>> +	total = dio_end - dio_start;
>> +	while (total) {
>> +		len = min_t(size_t, total, PAGE_SIZE);
>> +		bvec_set_page(&rqstp->rq_bvec[v], *(rqstp->rq_next_page++),
>> +			      len, 0);
>> +		total -= len;
>> +		++v;
>> +	}
>> +	WARN_ON_ONCE(v > rqstp->rq_maxpages);
> 
> I would rather we had an early test rather than a late warn-on.
> e.g.
>   if (total > (rqstp->rq_maxpages >> PAGE_SHIFT))
>      return -EINVAL /* or whatever */;
> 
> Otherwise it seems to be making unstated assumptions about how big the
> alignment requirements could be.

This is the same warn-on test that nfsd_iter_read does for buffered and
dontcache reads. It's done late because the final value of v is computed
here, not known before the loop.

I think we might be able to turn this into a short read, for all I/O
modes?


> Thanks,
> NeilBrown
> 
>> +
>> +	trace_nfsd_read_direct(rqstp, fhp, offset, *count);
>> +	iov_iter_bvec(&iter, ITER_DEST, rqstp->rq_bvec, v, dio_end - dio_start);
>> +
>> +	host_err = vfs_iocb_iter_read(nf->nf_file, &kiocb, &iter);
>> +	if (host_err >= 0) {
>> +		unsigned int pad = offset - dio_start;
>> +
>> +		/* The returned payload starts after the pad */
>> +		rqstp->rq_res.page_base = pad;
>> +
>> +		/* Compute the count of bytes to be returned */
>> +		if (host_err > pad + *count) {
>> +			host_err = *count;
>> +		} else if (host_err > pad) {
>> +			host_err -= pad;
>> +		} else {
>> +			host_err = 0;
>> +		}
>> +	} else if (unlikely(host_err == -EINVAL)) {
>> +		pr_info_ratelimited("nfsd: Unexpected direct I/O alignment failure\n");
>> +		host_err = -ESERVERFAULT;
>> +	}
>> +
>> +	return nfsd_finish_read(rqstp, fhp, nf->nf_file, offset, count,
>> +				eof, host_err);
>> +}
>> +
>>  /**
>>   * nfsd_iter_read - Perform a VFS read using an iterator
>>   * @rqstp: RPC transaction context
>> @@ -1106,6 +1179,11 @@ __be32 nfsd_iter_read(struct svc_rqst *rqstp, struct svc_fh *fhp,
>>  	switch (nfsd_io_cache_read) {
>>  	case NFSD_IO_BUFFERED:
>>  		break;
>> +	case NFSD_IO_DIRECT:
>> +		if (nf->nf_dio_read_offset_align && !base)
>> +			return nfsd_direct_read(rqstp, fhp, nf, offset,
>> +						count, eof);
>> +		fallthrough;
>>  	case NFSD_IO_DONTCACHE:
>>  		if (file->f_op->fop_flags & FOP_DONTCACHE)
>>  			kiocb.ki_flags = IOCB_DONTCACHE;
>> -- 
>> 2.50.0
>>
>>
>>
> 


-- 
Chuck Lever