Re: [PATCH v2 01/10] sunrpc: Remove backchannel check in svc_init_buffer()

[Jeff Layton <jlayton@kernel.org>]

On Sat, 2025-04-19 at 13:28 -0400, cel@kernel.org wrote:
> From: Chuck Lever <chuck.lever@oracle.com>
> 
> A backchannel service might use forechannel send and receive
> buffers, but svc_recv() still calls svc_alloc_arg() on backchannel
> svc_rqsts, so it will populate the svc_rqst's rq_pages[] array
> anyway. The "is backchannel" check in svc_init_buffer() saves us
> nothing.
> 
> In a moment, I plan to replace the rq_pages[] array with a
> dynamically-allocated piece of memory, and svc_init_buffer() is
> where that memory will get allocated. Backchannel requests actually
> do use that array, so it has to be available to handle those
> requests without a segfault.
> 
> XXX: Or, make svc_alloc_arg() ignore backchannel requests too?
>      Could set rqstp->rq_maxpages to zero.
> 

Maybe I'm confused here, but the backchannel still needs some pages to
do its thing? I guess the main change here is that the pages are
allocated at svc_create time instead of waiting until later?

> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> ---
>  net/sunrpc/svc.c | 4 ----
>  1 file changed, 4 deletions(-)
> 
> diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
> index e7f9c295d13c..8ce3e6b3df6a 100644
> --- a/net/sunrpc/svc.c
> +++ b/net/sunrpc/svc.c
> @@ -640,10 +640,6 @@ svc_init_buffer(struct svc_rqst *rqstp, unsigned int size, int node)
>  {
>  	unsigned long pages, ret;
>  
> -	/* bc_xprt uses fore channel allocated buffers */
> -	if (svc_is_backchannel(rqstp))
> -		return true;
> -
>  	pages = size / PAGE_SIZE + 1; /* extra page as we hold both request and reply.
>  				       * We assume one is at most one page
>  				       */

Acked-by: Jeff Layton <jlayton@kernel.org>