Re: [PATCH net] Fix clamp() of ip_vs_conn_tab on small memory systems.

[Julian Anastasov <ja@ssi.bg>]

	Hello,

On Fri, 6 Dec 2024, David Laight wrote:

> The intention of the code seems to be that the minimum table
> size should be 256 (1 << min).
> However the code uses max = clamp(20, 5, max_avail) which implies

	Actually, it tries to reduce max=20 (max possible) below
max_avail: [8 .. max_avail]. Not sure what 5 is here...

> the author thought max_avail could be less than 5.
> But clamp(val, min, max) is only well defined for max >= min.
> If max < min whether is returns min or max depends on the order of
> the comparisons.

	Looks like max_avail goes below 8 ? What value you see
for such small system?

> Change to clamp(max_avail, 5, 20) which has the expected behaviour.

	It should be clamp(max_avail, 8, 20)

> 
> Replace the clamp_val() on the line below with clamp().
> clamp_val() is just 'an accident waiting to happen' and not needed here.

	OK

> Fixes: 4f325e26277b6
> (Although I actually doubt the code is used on small memory systems.)
> 
> Detected by compile time checks added to clamp(), specifically:
> minmax.h: use BUILD_BUG_ON_MSG() for the lo < hi test in clamp()

	Existing or new check? Does it happen that max_avail
is a constant, so that a compile check triggers?

> 
> Signed-off-by: David Laight <david.laight@aculab.com>

	The code below looks ok to me but can you change the
comments above to more correctly specify the values and if the
problem is that max_avail goes below 8 (min).

> ---
>  net/netfilter/ipvs/ip_vs_conn.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/net/netfilter/ipvs/ip_vs_conn.c b/net/netfilter/ipvs/ip_vs_conn.c
> index 98d7dbe3d787..c0289f83f96d 100644
> --- a/net/netfilter/ipvs/ip_vs_conn.c
> +++ b/net/netfilter/ipvs/ip_vs_conn.c
> @@ -1495,8 +1495,8 @@ int __init ip_vs_conn_init(void)
>  	max_avail -= 2;		/* ~4 in hash row */
>  	max_avail -= 1;		/* IPVS up to 1/2 of mem */
>  	max_avail -= order_base_2(sizeof(struct ip_vs_conn));

	More likely we can additionally clamp max_avail here:

	max_avail = max(min, max_avail);

	But your solution solves the problem with less lines.

> -	max = clamp(max, min, max_avail);
> -	ip_vs_conn_tab_bits = clamp_val(ip_vs_conn_tab_bits, min, max);
> +	max = clamp(max_avail, min, max);
> +	ip_vs_conn_tab_bits = clamp(ip_vs_conn_tab_bits, min, max);
>  	ip_vs_conn_tab_size = 1 << ip_vs_conn_tab_bits;
>  	ip_vs_conn_tab_mask = ip_vs_conn_tab_size - 1;
>  
> -- 
> 2.17.1

Regards

--
Julian Anastasov <ja@ssi.bg>