netdev - Re: [PATCH net-next] loopback: clear pfmemalloc on outgoing skb's

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Date:   Wed, 01 Feb 2017 15:38:05 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Josef Bacik <jbacik@...com>
Cc:     netdev@...r.kernel.org, kernel-team@...com,
        "David S . Miller" <davem@...emloft.net>
Subject: Re: [PATCH net-next] loopback: clear pfmemalloc on outgoing skb's

On Wed, 2017-02-01 at 16:04 -0500, Josef Bacik wrote:
> I was seeing random disconnects while testing NBD over loopback.  This turned
> out to be because NBD sets pfmemalloc on it's socket, however the receiving side
> is a user space application so does not have pfmemalloc set on its socket.  This
> means that sk_filter_trim_cap will simply drop this packet, under the assumption
> that the other side will simply retransmit.  Well we do retransmit, and then the
> packet is just dropped again for the same reason.  To keep this from happening
> simply clear skb->pfmemalloc on transmit so that we don't drop the packet on the
> receive side.
> 
> Signed-off-by: Josef Bacik <jbacik@...com>
> ---
>  drivers/net/loopback.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/net/loopback.c b/drivers/net/loopback.c
> index 1e05b7c..13c9126 100644
> --- a/drivers/net/loopback.c
> +++ b/drivers/net/loopback.c
> @@ -81,6 +81,13 @@ static netdev_tx_t loopback_xmit(struct sk_buff *skb,
>  	 */
>  	skb_dst_force(skb);
>  
> +	/* If our transmitter was a pfmemalloc socket we need to clear
> +	 * pfmemalloc here, otherwise the receiving socket may not be
> +	 * pfmemalloc, and if this is a tcp packet then it'll get dropped and
> +	 * all traffic will halt.
> +	 */
> +	skb->pfmemalloc = false;
> +

I am not sure this is a proper fix.

Presumably if the socket was able to store packets in its write queue,
fact that it sends it to loopback or an Ethernet link should not matter.

Only in RX path the pfmemalloc thing is really important.

So I would rather not set skb->pfmemalloc for skbs allocated for the
write queue, and more exactly the fast clone.

This would actually speed up the stack a bit.

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 734c71468b013838516cfe8c744dcd0e797a6e2b..f91b81340dc5be80e0c26f9835d9192f35b75ad7 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -271,7 +271,7 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
                atomic_set(&fclones->fclone_ref, 1);
 
                fclones->skb2.fclone = SKB_FCLONE_CLONE;
-               fclones->skb2.pfmemalloc = pfmemalloc;
+               fclones->skb2.pfmemalloc = 0;
        }
 out:
        return skb;