linux-kernel - Re: [PATCH net-next v3 5/5] net: Document memory provider driver support

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Message-ID: <20241213175527.01583db8@kernel.org>
Date: Fri, 13 Dec 2024 17:55:27 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: Mina Almasry <almasrymina@...gle.com>
Cc: netdev@...r.kernel.org, Pavel Begunkov <asml.silence@...il.com>, Kaiyuan
 Zhang <kaiyuanz@...gle.com>, Willem de Bruijn <willemb@...gle.com>,
 Samiullah Khawaja <skhawaja@...gle.com>, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, "David S. Miller" <davem@...emloft.net>, Eric
 Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, Simon
 Horman <horms@...nel.org>, Jonathan Corbet <corbet@....net>, Jesper
 Dangaard Brouer <hawk@...nel.org>, Ilias Apalodimas
 <ilias.apalodimas@...aro.org>
Subject: Re: [PATCH net-next v3 5/5] net: Document memory provider driver
 support

On Fri, 13 Dec 2024 09:50:15 -0800 Mina Almasry wrote:
> > No? It should all just work. The page may get split / fragmented by
> > the driver or page_pool_alloc_netmem() which you're adding in this
> > series. A fragmented net_iov will have an elevated refcount in exactly
> > the same way as if the driver was stashing one ref to release later.  
> 
> Ah, I had not considered that the driver may recycle the page by
> holding onto a pp ref count, rather than a page refcount. The former
> should indeed just work, although I don't have a driver that does
> this, so test coverage may be a bit lacking.
> 
> But you mentioned you like the rule above. Do you want this removed
> from the docs entirely? Or clarified to something like "driver's can't
> perform their own recycling by holding onto page refs, but can hold
> onto page_pool refs"?

We can call it out, makes sense. I'm not sure how to clearly name 
the page ref vs page_pool ref. But yes, driver must not attempt to
hold onto struct page for recycling, as there is no struct page.

I'd add to that that recycling using page pool refs is also discouraged
as the circulation time for buffers is much longer than when data is
copied out during recvmsg().