Re: [PATCH 11/17] netlink: Clearer reasoning about the netlink response buffer size

[Stefano Brivio <sbrivio@redhat.com>]

On Mon, 24 Jul 2023 16:09:30 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> Currently we set NLBUFSIZ large enough for 8192 netlink headers (128kiB in
> total), and reference netlink(7).  However netlink(7) says nothing about
> reponse buffer sizes, and the documents which do reference 8192 *bytes* not
> 8192 headers.

Oops.

> Update NLBUFSIZ to 64kiB with a more detailed rationale.
> 
> Link: https://bugs.passt.top/show_bug.cgi?id=67
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  netlink.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/netlink.c b/netlink.c
> index cdd65c0..d553ddd 100644
> --- a/netlink.c
> +++ b/netlink.c
> @@ -35,7 +35,14 @@
>  #include "log.h"
>  #include "netlink.h"
>  
> -#define NLBUFSIZ	(8192 * sizeof(struct nlmsghdr)) /* See netlink(7) */
> +/* Netlink expects a buffer of at least 8kiB or the system page size,
> + * whichever is larger.  32kiB is recommended for more efficient.
> + * Since the largest page size on any remotely common Linux setup is
> + * 64kiB (ppc64), that should cover it.
> + *
> + * https://www.kernel.org/doc/html/next/userspace-api/netlink/intro.html#buffer-sizing
> + */
> +#define NLBUFSIZ 65536

I'm fine with this, but we also have PAGE_SIZE and MAX() defined. Or
maybe it's more reasonable to keep this constant. I'm not sure.

-- 
Stefano