Re: [PATCH bpf] xsk: Add truesize to skb_add_rx_frag().

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Fri, Feb 02, 2024 at 05:32:20PM +0100, Sebastian Andrzej Siewior wrote:
> xsk_build_skb() allocates a page and adds it to the skb via
> skb_add_rx_frag() and specifies 0 for truesize. This leads to a warning
> in skb_add_rx_frag() with CONFIG_DEBUG_NET enabled because size is
> larger than truesize.
> 
> Increasing truesize requires to add the same amount to socket's
> sk_wmem_alloc counter in order not to underflow the counter during
> release in the destructor (sock_wfree()).
> 
> Pass the size of the allocated page as truesize to skb_add_rx_frag().
> Add this mount to socket's sk_wmem_alloc counter.
> 
> Fixes: cf24f5a5feea ("xsk: add support for AF_XDP multi-buffer on Tx path")
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Thanks a lot! Of course fix is totally correct.

Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>

> ---
> Noticed by running test_xsk.sh
> 
>  net/xdp/xsk.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index 0348e4bde23b2..3050739cfe1e0 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -744,7 +744,8 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
>  			memcpy(vaddr, buffer, len);
>  			kunmap_local(vaddr);
>  
> -			skb_add_rx_frag(skb, nr_frags, page, 0, len, 0);
> +			skb_add_rx_frag(skb, nr_frags, page, 0, len, PAGE_SIZE);
> +			refcount_add(PAGE_SIZE, &xs->sk.sk_wmem_alloc);
>  		}
>  
>  		if (first_frag && desc->options & XDP_TX_METADATA) {
> -- 
> 2.43.0
>