Re: [PATCH 1/2] sunrpc: add an extra reserve page to svc_serv_maxpages()

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

On 9/9/25 7:33 PM, Mike Snitzer wrote:
> nfsd_iter_read() might need two extra pages when a READ payload is not
> DIO-aligned -- but nfsd_iter_read() and nfsd_splice_actor() are
> mutually exclusive (so reuse page reserved for nfsd_splice_actor).
> 
> Signed-off-by: Mike Snitzer <snitzer@kernel.org>
> ---
>  include/linux/sunrpc/svc.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
> index e64ab444e0a7f..190c2667500e2 100644
> --- a/include/linux/sunrpc/svc.h
> +++ b/include/linux/sunrpc/svc.h
> @@ -163,10 +163,13 @@ extern u32 svc_max_payload(const struct svc_rqst *rqstp);
>   * pages, one for the request, and one for the reply.
>   * nfsd_splice_actor() might need an extra page when a READ payload
>   * is not page-aligned.
> + * nfsd_iter_read() might need two extra pages when a READ payload
> + * is not DIO-aligned -- but nfsd_iter_read() and nfsd_splice_actor()
> + * are mutually exclusive (so reuse page reserved for nfsd_splice_actor).
>   */
>  static inline unsigned long svc_serv_maxpages(const struct svc_serv *serv)
>  {
> -	return DIV_ROUND_UP(serv->sv_max_mesg, PAGE_SIZE) + 2 + 1;
> +	return DIV_ROUND_UP(serv->sv_max_mesg, PAGE_SIZE) + 2 + 1 + 1;
>  }
>  
>  /*

I'm not convinced an additional page is necessary. The most the direct
I/O range can be is rsize + 1 page, AFAICT, and that seems to be covered
already.

If we want NFSD to support file systems / block devices with alignment
requirements large than a page, then perhaps additional buffer space
will be needed... or NFSD can return a short read.

-- 
Chuck Lever