netdev - Re: [PATCH net] Revert "virtio_net: replace netdev_alloc_skb_ip_align() with napi_alloc

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Date:   Wed, 13 Jan 2021 16:44:46 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Greg Thelen <gthelen@...gle.com>
Subject: Re: [PATCH net] Revert "virtio_net: replace
 netdev_alloc_skb_ip_align() with napi_alloc_skb()"

On Tue, 2021-01-12 at 21:12 -0800, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@...gle.com>
> 
> This reverts commit c67f5db82027ba6d2ea4ac9176bc45996a03ae6a.
> 
> While using page fragments instead of a kmalloc backed skb->head might give
> a small performance improvement in some cases, there is a huge risk of
> memory use under estimation.
> 
> GOOD_COPY_LEN is 128 bytes. This means that we need a small amount
> of memory to hold the headers and struct skb_shared_info
> 
> Yet, napi_alloc_skb() might use a whole 32KB page (or 64KB on PowerPc)
> for long lived incoming TCP packets.
> 
> We have been tracking OOM issues on GKE hosts hitting tcp_mem limits
> but consuming far more memory for TCP buffers than instructed in tcp_mem[2]
> 
> Even if we force napi_alloc_skb() to only use order-0 pages, the issue
> would still be there on arches with PAGE_SIZE >= 32768
> 
> Using alloc_skb() and thus standard kmallloc() for skb->head allocations
> will get the benefit of letting other objects in each page being independently
> used by other skbs, regardless of the lifetime.
> 
> Note that a similar problem exists for skbs allocated from napi_get_frags(),
> this is handled in a separate patch.
> 
> I would like to thank Greg Thelen for his precious help on this matter,
> analysing crash dumps is always a time consuming task.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Paolo Abeni <pabeni@...hat.com>
> Cc: Michael S. Tsirkin <mst@...hat.com>
> Cc: Greg Thelen <gthelen@...gle.com>
> ---
>  drivers/net/virtio_net.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 508408fbe78fbd8658dc226834b5b1b334b8b011..5886504c1acacf3f6148127b5c1cc7f6a906b827 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -386,7 +386,7 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
>  	p = page_address(page) + offset;
>  
>  	/* copy small packet so we can reuse these pages for small data */
> -	skb = napi_alloc_skb(&rq->napi, GOOD_COPY_LEN);
> +	skb = netdev_alloc_skb_ip_align(vi->dev, GOOD_COPY_LEN);
>  	if (unlikely(!skb))
>  		return NULL;

I'm ok with the revert. The gain given by the original change in my
tests was measurable, but small.

Acked-by: Paolo Abeni <pabeni@...hat.com>