Re: [PATCH mptcp-net] mptcp: try harder to borrow memory from subflow under pressure

[Mat Martineau <mathew.j.martineau@linux.intel.com>]

On Wed, 26 May 2021, Paolo Abeni wrote:

> If the host is under sever memory pressure, and RX forward
> memory allocation for the msk fails, we try to borrow the
> required memory from the ingress subflow.
>
> The current attempt is a bit flaky: if skb->truesize is less
> than SK_MEM_QUANTUM, the ssk will not release any memory, and
> the next schedule will fail again.
>
> Instead, directly move the required amount of pages from the
> ssk to the msk, if available
>
> Fixes: 9c3f94e1681b ("mptcp: add missing memory scheduling in the rx path")
> Signed-off-by: Paolo Abeni <pabeni@redhat.com>
> ---
> net/mptcp/protocol.c | 10 ++++++----
> 1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
> index bb029dd4ff5e..785e74c13b3c 100644
> --- a/net/mptcp/protocol.c
> +++ b/net/mptcp/protocol.c
> @@ -286,11 +286,13 @@ static bool __mptcp_move_skb(struct mptcp_sock *msk, struct sock *ssk,
>
> 	/* try to fetch required memory from subflow */
> 	if (!sk_rmem_schedule(sk, skb, skb->truesize)) {
> -		if (ssk->sk_forward_alloc < skb->truesize)
> -			goto drop;
> -		__sk_mem_reclaim(ssk, skb->truesize);
> -		if (!sk_rmem_schedule(sk, skb, skb->truesize))
> +		int amount = sk_mem_pages(skb->truesize) << SK_MEM_QUANTUM_SHIFT;
> +
> +		if (ssk->sk_forward_alloc < amount)
> 			goto drop;
> +
> +		ssk->sk_forward_alloc -= amount;
> +		sk->sk_forward_alloc += amount;
> 	}
>
> 	has_rxtstamp = TCP_SKB_CB(skb)->has_rxtstamp;
> -- 
> 2.26.3

Looks ok to me - the memory will be reclaimed from the msk later, if I 
understand correctly.

Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>

--
Mat Martineau
Intel