netdev - Re: [PATCH net-next] xdp: add multi-buff support for xdp running in generic mode

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Date: Tue, 28 Nov 2023 15:10:28 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: Lorenzo Bianconi <lorenzo@...nel.org>
Cc: Lorenzo Bianconi <lorenzo.bianconi@...hat.com>, netdev@...r.kernel.org,
 davem@...emloft.net, edumazet@...gle.com, pabeni@...hat.com,
 bpf@...r.kernel.org, hawk@...nel.org, toke@...hat.com
Subject: Re: [PATCH net-next] xdp: add multi-buff support for xdp running in
 generic mode

On Tue, 28 Nov 2023 23:27:29 +0100 Lorenzo Bianconi wrote:
> > Yes, don't we allow writes to fragments in XDP based on the assumption
> > that it runs on Rx so that paged data must not be zero copy?
> > bpf_xdp_store_bytes() doesn't seem to have any checks which would
> > stop it from writing fragments, as far as I can see.  
> 
> do you mean in the skb use-case we could write to fragments (without copying
> them) if the skb is not cloned and the paged area is not 'zero-copied'?

The zero-copy thing is a red herring. If application uses
sendpage/sendfile/splice the frag may be a page cache page
of a file. Or something completely read only.

IIUC you're trying to avoid the copy if the prog is mbuf capable.
So I was saying that can't work for forms of XDP which actually 
deal with skbs. But that wasn't really your question, sorry :)

> With respect to this patch it would mean we can rely on pskb_expand_head() to
> reallocate the skb and to covert it to a xdp_buff and we do not need to explicitly
> reallocate fragments as we currently do for veth in veth_convert_skb_to_xdp_buff() [0].
> Is my understanding correct or am I missing something?

The difference is that pskb_expand_head() will give you a linear skb,
potentially triggering an order 5 allocation. Expensive and likely to
fail under memory pressure.

veth_convert_skb_to_xdp_buff() tries to allocate pages, and keep
the skb fragmented.