SUNRPC: RPC buffer size estimates are too large

The RPC buffer size estimation logic in net/sunrpc/clnt.c always
significantly overestimates the requirements for the buffer size.
A little instrumentation demonstrated that in fact rpc_malloc was never
allocating the buffer from the mempool, but almost always called kmalloc.

To compute the size of the RPC buffer more precisely, split p_bufsiz into
two fields; one for the argument size, and one for the result size.

Then, compute the sum of the exact call and reply header sizes, and split
the RPC buffer precisely between the two.  That should keep almost all RPC
buffers within the 2KiB buffer mempool limit.

And, we can finally be rid of RPC_SLACK_SPACE!

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

1 files changed, 5 insertions, 2 deletions

diff --git a/fs/nfs/mount_clnt.c b/fs/nfs/mount_clnt.c
index f75fe72b4160..ca5a266a3140 100644
--- a/fs/nfs/mount_clnt.c
+++ b/fs/nfs/mount_clnt.c
@@ -133,13 +133,15 @@ xdr_decode_fhstatus3(struct rpc_rqst *req, __be32 *p, struct mnt_fhstatus *res)
 
 #define MNT_dirpath_sz          (1 + 256)
 #define MNT_fhstatus_sz         (1 + 8)
+#define MNT_fhstatus3_sz        (1 + 16)
 
 static struct rpc_procinfo      mnt_procedures[] = {
 [MNTPROC_MNT] = {
           .p_proc               = MNTPROC_MNT,
           .p_encode             = (kxdrproc_t) xdr_encode_dirpath,      
           .p_decode             = (kxdrproc_t) xdr_decode_fhstatus,
-          .p_bufsiz             = MNT_dirpath_sz << 2,
+          .p_arglen             = MNT_dirpath_sz,
+          .p_replen             = MNT_fhstatus_sz,
           .p_statidx            = MNTPROC_MNT,
           .p_name               = "MOUNT",
         },
@@ -150,7 +152,8 @@ static struct rpc_procinfo mnt3_procedures[] = {
           .p_proc               = MOUNTPROC3_MNT,
           .p_encode             = (kxdrproc_t) xdr_encode_dirpath,
           .p_decode             = (kxdrproc_t) xdr_decode_fhstatus3,
-          .p_bufsiz             = MNT_dirpath_sz << 2,
+          .p_arglen             = MNT_dirpath_sz,
+          .p_replen             = MNT_fhstatus3_sz,
           .p_statidx            = MOUNTPROC3_MNT,
           .p_name               = "MOUNT",
         },