From: Chuck Lever <[email protected]>
We've discovered that delivering a CB_OFFLOAD operation can be
unreliable in some pretty unremarkable situations. Examples
include:
- The server dropped the connection because it lost a forechannel
NFSv4 request and wishes to force the client to retransmit
- The GSS sequence number window under-flowed
- A network partition occurred
When that happens, all pending callback operations, including
CB_OFFLOAD, are lost. NFSD does not retransmit them.
Moreover, the Linux NFS client does not yet support sending an
OFFLOAD_STATUS operation to probe whether an asynchronous COPY
operation has finished. Thus, on Linux NFS clients, when a
CB_OFFLOAD is lost, asynchronous COPY can hang until manually
interrupted.
I've tried a couple of remedies, but so far the side-effects are
worse than the disease and they have had to be reverted. So
temporarily force COPY operations to be synchronous so that the use
of CB_OFFLOAD is avoided entirely. This is a fix that can easily be
backported to LTS kernels. I am working on client patches that
introduce an implementation of OFFLOAD_STATUS.
Note that NFSD arbitrarily limits the size of a copy_file_range
to 4MB to avoid indefinitely blocking an nfsd thread. A short
COPY result is returned in that case, and the client can present
a fresh COPY request for the remainder.
Signed-off-by: Chuck Lever <[email protected]>
---
fs/nfsd/nfs4proc.c | 7 +++++++
1 file changed, 7 insertions(+)
Changes since v1:
- Clarify that this patch is for backporting, and a longer-term
fix is in the works for subsequent upstream kernels
- Note that synchronous COPY operations don't indefinitely block
nfsd threads
diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
index ea3cc3e870a7..46bd20fe5c0f 100644
--- a/fs/nfsd/nfs4proc.c
+++ b/fs/nfsd/nfs4proc.c
@@ -1807,6 +1807,13 @@ nfsd4_copy(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
__be32 status;
struct nfsd4_copy *async_copy = NULL;
+ /*
+ * Currently, async COPY is not reliable. Force all COPY
+ * requests to be synchronous to avoid client application
+ * hangs waiting for COPY completion.
+ */
+ nfsd4_copy_set_sync(copy, true);
+
copy->cp_clp = cstate->clp;
if (nfsd4_ssc_is_inter(copy)) {
trace_nfsd_copy_inter(copy);
base-commit: 939cb14d51a150e3c12ef7a8ce0ba04ce6131bd2
--
2.44.0