On 9/9/25 7:37 PM, NeilBrown wrote:
> On Wed, 10 Sep 2025, Chuck Lever wrote:
>> From: Chuck Lever <chuck.lever@xxxxxxxxxx>
>>
>> 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@xxxxxxxxxx>
>> Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
>> ---
>> 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
