[RFC PATCH v3 4/3] NFSD: Implement NFSD_IO_DIRECT for NFS READ (v3 and older) [Was: "Re: [PATCH v3 3/3] NFSD: Implement NFSD_IO_DIRECT for NFS READ"]

[Mike Snitzer <snitzer@kernel.org>]

On Wed, Sep 24, 2025 at 11:18:53AM -0400, Mike Snitzer wrote:
> 
> FYI, I've been testing with NFSv3.  So nfsd_read -> nfsd_iter_read
> 
> The last time I tested NFSv4 was with my DIO READ code that checked
> the memory alignment with precision (albeit adding more work per IO,
> which Chuck would like to avoid).

We could just enable NFSD_IO_DIRECT for NFS v3 and older. And work
out adding support for NFSv4 as follow-on work for next cycle?

This could be folded into Patch 3/3, and its header updated to be
clear that NFS v4+ isn't supported yet:

diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index 96792f8a8fc5a..459a29f377c2c 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -1182,10 +1182,6 @@ __be32 nfsd_iter_read(struct svc_rqst *rqstp, struct svc_fh *fhp,
 	case NFSD_IO_BUFFERED:
 		break;
 	case NFSD_IO_DIRECT:
-		if (nf->nf_dio_read_offset_align &&
-		    !rqstp->rq_res.page_len && !base)
-			return nfsd_direct_read(rqstp, fhp, nf, offset,
-						count, eof);
 		fallthrough;
 	case NFSD_IO_DONTCACHE:
 		if (file->f_op->fop_flags & FOP_DONTCACHE)
@@ -1622,8 +1618,14 @@ __be32 nfsd_read(struct svc_rqst *rqstp, struct svc_fh *fhp,
 	file = nf->nf_file;
 	if (file->f_op->splice_read && nfsd_read_splice_ok(rqstp))
 		err = nfsd_splice_read(rqstp, fhp, file, offset, count, eof);
-	else
+	else {
+		/* Use vectored reads, for NFS v3 and older */
+		if (nfsd_io_cache_read == NFSD_IO_DIRECT &&
+		    nf->nf_dio_read_offset_align &&
+		    !rqstp->rq_res.page_len && !base)
+			return nfsd_direct_read(rqstp, fhp, nf, offset, count, eof);
 		err = nfsd_iter_read(rqstp, fhp, nf, offset, count, 0, eof);
+	}
 
 	nfsd_file_put(nf);
 	trace_nfsd_read_done(rqstp, fhp, offset, *count);