netdev - Re: [PATCH net] bnxt_en: do not map packet buffers twice

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Date: Fri, 15 Dec 2023 11:57:14 -0500
From: Andy Gospodarek <andrew.gospodarek@...adcom.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Michael Chan <michael.chan@...adcom.com>,
	Andy Gospodarek <andrew.gospodarek@...adcom.com>,
	davem@...emloft.net, netdev@...r.kernel.org, edumazet@...gle.com,
	pabeni@...hat.com, bpf@...r.kernel.org, hawk@...nel.org,
	ast@...nel.org, daniel@...earbox.net, john.fastabend@...il.com,
	Somnath Kotur <somnath.kotur@...adcom.com>
Subject: Re: [PATCH net] bnxt_en: do not map packet buffers twice

On Fri, Dec 15, 2023 at 08:37:59AM -0800, Jakub Kicinski wrote:
> On Thu, 14 Dec 2023 13:31:38 -0800 Michael Chan wrote:
> > From: Andy Gospodarek <andrew.gospodarek@...adcom.com>
> > 
> > Remove double-mapping of DMA buffers as it can prevent page pool entries
> > from being freed.  Mapping is managed by page pool infrastructure and
> > was previously managed by the driver in __bnxt_alloc_rx_page before
> > allowing the page pool infrastructure to manage it.
> 
> This patch is all good, but I'm confused by the handling of head.
> Do you recycle it immediately and hope that the Tx happens before
> the Rx gets around to using the recycled page again? Am I misreading?

Your description is correct, but we use a better strategy that just
hoping it works out. :)

The design is that we do not update the rx ring with the producer value
that was present when the packet was received until after getting the tx
completion indicating that the packet sent via XDP_TX action has been
sent.