[RFC PATCH] NFSD: Add a "file_sync" export option

[Chuck Lever <cel@kernel.org>]

From: Chuck Lever <chuck.lever@oracle.com>

Introduce the kernel pieces for a "file_sync" export option. This
option would make all NFS WRITE operations on one export a
FILE_SYNC WRITE.

There are two primary use cases for this new export option:

1. The exported file system is not backed by persistent storage.
   Thus a subsequent COMMIT will be a no-op. To prevent the client
   from wasting the extra round-trip on a COMMIT operation, convert
   all WRITEs to files on that export to FILE_SYNC.

2. The exported file system is backed by persistent storage that is
   faster than the mean network round trip with the client. Waiting
   for a separate COMMIT operation would cost more time than just
   committing the data during the WRITE operation.

   Either the underlying persistent storage is faster than most any
   network fabric (eg, NVMe); or the network connection to the
   client is very high latency (eg, a WAN link).

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 fs/nfsd/export.c                 | 1 +
 fs/nfsd/nfs4proc.c               | 1 +
 fs/nfsd/vfs.c                    | 5 +++--
 include/uapi/linux/nfsd/export.h | 3 ++-
 4 files changed, 7 insertions(+), 3 deletions(-)

This patch is a year old, so won't apply to current kernels. But
the idea is similar to Mike's suggestion that NFSD_IO_DIRECT
should promote all NFS WRITEs to durable writes, but is much
simpler in execution. Any interest in revisiting this approach?

diff --git a/fs/nfsd/export.c b/fs/nfsd/export.c
index c82d8e3e0d4f..11b5337dd0ea 100644
--- a/fs/nfsd/export.c
+++ b/fs/nfsd/export.c
@@ -1297,6 +1297,7 @@ static struct flags {
 	{ NFSEXP_V4ROOT, {"v4root", ""}},
 	{ NFSEXP_PNFS, {"pnfs", ""}},
 	{ NFSEXP_SECURITY_LABEL, {"security_label", ""}},
+	{ NFSEXP_FILE_SYNC, {"file_sync", ""}},
 	{ 0, {"", ""}}
 };
 
diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
index 51bae11d5d23..7a4ded3ff7c2 100644
--- a/fs/nfsd/nfs4proc.c
+++ b/fs/nfsd/nfs4proc.c
@@ -1269,6 +1269,7 @@ nfsd4_clone(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 	status = nfsd4_clone_file_range(rqstp, src, clone->cl_src_pos,
 			dst, clone->cl_dst_pos, clone->cl_count,
 			EX_ISSYNC(cstate->current_fh.fh_export));
+	/* cel: check the "file_sync" export option as well */
 
 	nfsd_file_put(dst);
 	nfsd_file_put(src);
diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index cd00d95c997f..ffa6db6851bd 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -1205,9 +1205,10 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfsd_file *nf,
 
 	exp = fhp->fh_export;
 
-	if (!EX_ISSYNC(exp))
+	if (exp->ex_flags & NFSEXP_FILE_SYNC)
+		*stable = NFS_FILE_SYNC;
+	else if (!EX_ISSYNC(exp))
 		*stable = NFS_UNSTABLE;
-
 	if (*stable && !fhp->fh_use_wgather)
 		flags |= RWF_SYNC;
 
diff --git a/include/uapi/linux/nfsd/export.h b/include/uapi/linux/nfsd/export.h
index a73ca3703abb..45afec454a37 100644
--- a/include/uapi/linux/nfsd/export.h
+++ b/include/uapi/linux/nfsd/export.h
@@ -53,9 +53,10 @@
  */
 #define	NFSEXP_V4ROOT		0x10000
 #define NFSEXP_PNFS		0x20000
+#define NFSEXP_FILE_SYNC	0x40000
 
 /* All flags that we claim to support.  (Note we don't support NOACL.) */
-#define NFSEXP_ALLFLAGS		0x3FEFF
+#define NFSEXP_ALLFLAGS		0x7FEFF
 
 /* The flags that may vary depending on security flavor: */
 #define NFSEXP_SECINFO_FLAGS	(NFSEXP_READONLY | NFSEXP_ROOTSQUASH \
-- 
2.51.0