[PATCH, RFC] backchannel overflows

[Christoph Hellwig <hch@infradead.org>]

Currently the client will just crap out if a CB_NULL comes in at the
same time as a slot controlled CB_COMPOUND that includes a CB_SEQUENCE.

I don't really understand how the spec wants to treat CB_NULL in
relation to the available backchannel slots, but given that it's
not part of the sequences in CB_SEQUENCE it somehow nees to bypass them.

If we make sure to overallocate the rpc-level buffers so that we
have more than the available NFS-level slots we should be safe from
this condition which makes a 4.1 server doing heavy recalls under
load very unhapy by not returning an NFS level reply to its layout
recalls.

I dont really like this patch much, so any idea for a better solution
would be highly welcome!

diff --git a/fs/nfs/callback.h b/fs/nfs/callback.h
index 84326e9..7afb3ef 100644
--- a/fs/nfs/callback.h
+++ b/fs/nfs/callback.h
@@ -205,7 +205,7 @@ extern int nfs4_set_callback_sessionid(struct nfs_client *clp);
  * so we limit their concurrency to 1 by setting up the maximum number
  * of slots for the backchannel.
  */
-#define NFS41_BC_MIN_CALLBACKS 1
+#define NFS41_BC_MIN_CALLBACKS 2
 #define NFS41_BC_MAX_CALLBACKS 1
 
 extern unsigned int nfs_callback_set_tcpport;