netdev - Re: [PATCH 3/4] gve: Rx Buffer Recycling

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Date:   Tue, 03 Nov 2020 16:01:38 -0800
From:   Saeed Mahameed <saeed@...nel.org>
To:     David Awogbemila <awogbemila@...gle.com>, netdev@...r.kernel.org
Subject: Re: [PATCH 3/4] gve: Rx Buffer Recycling

On Tue, 2020-11-03 at 09:46 -0800, David Awogbemila wrote:
> This patch lets the driver reuse buffers that have been freed by the
> networking stack.
> 
> In the raw addressing case, this allows the driver avoid allocating
> new
> buffers.
> In the qpl case, the driver can avoid copies.
> 
> Signed-off-by: David Awogbemila <awogbemila@...gle.com>
> ---
>  drivers/net/ethernet/google/gve/gve.h    |  10 +-
>  drivers/net/ethernet/google/gve/gve_rx.c | 194 +++++++++++++++----
> ----

> +	if (len <= priv->rx_copybreak) {
> +		/* Just copy small packets */
> +		skb = gve_rx_copy(dev, napi, page_info, len);
> +		u64_stats_update_begin(&rx->statss);
> +		rx->rx_copied_pkt++;
> +		rx->rx_copybreak_pkt++;
> +		u64_stats_update_end(&rx->statss);
> +	} else {
> +		bool can_flip = gve_rx_can_flip_buffers(dev);
> +		int recycle = 0;
> +
> +		if (can_flip) {
> +			recycle = gve_rx_can_recycle_buffer(page_info-
> >page);
> +			if (recycle < 0) {
> +				gve_schedule_reset(priv);

How would a reset solve anything if your driver is handling pages with
"bad" refcount, i don't agree here that reset is the best course of
action, all you can do here is warn and leak the page ...
this is a critical driver bug and not something that user should
expect. 

> +	
> +		} else {
> +			/* It is possible that the networking stack has
> already
> +			 * finished processing all outstanding packets
> in the buffer
> +			 * and it can be reused.
> +			 * Flipping is unnecessary here - if the
> networking stack still
> +			 * owns half the page it is impossible to tell
> which half. Either
> +			 * the whole page is free or it needs to be
> replaced.
> +			 */
> +			int recycle =
> gve_rx_can_recycle_buffer(page_info->page);
> +
> +			if (recycle < 0) {
> +				gve_schedule_reset(priv);
> +				return false;

Same.