Re: [PATCH, RFC] backchannel overflows

[Chuck Lever <chuck.lever@oracle.com>]

On Apr 28, 2015, at 4:21 PM, Christoph Hellwig <hch@infradead.org> 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.

Under what circumstances does the server send a CB_NULL while a CB_COMPOUND
is in flight?


> 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;
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--
Chuck Lever
chuck[dot]lever[at]oracle[dot]com