Re: [PATCH, RFC] backchannel overflows

[Kinglong Mee <kinglongmee@gmail.com>]

On 4/29/2015 4:21 AM, Christoph Hellwig wrote:
> 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

Are you sure that's okay without update NFS41_BC_MAX_CALLBACKS?

thanks,
Kinglong Mee