From: Eric Dumazet

Sent: 14 January 2021 05:17

On Wed, Jan 13, 2021 at 11:23 PM David Laight [off-list ref] wrote:

quoted

From: Eric Dumazet

quoted

Sent: 13 January 2021 16:18

From: Eric Dumazet <edumazet@google.com>

Both virtio net and napi_get_frags() allocate skbs
with a very small skb->head

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
under estimating memory usage.

There is (or was last time I looked) also a problem with
some of the USB ethernet drivers.

IIRC one of the ASXnnnnnn (???) USB3 ones allocates 64k skb to pass
to the USB stack and then just lies about skb->truesize when passing
them into the network stack.

You sure ? I think I have fixed this at some point

https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git/commit/?id=a9e0aca4b37885b5599e52211f09
8bd7f565e749

I might have forgotten that patch :-)
Or possibly only remembered it changing small packets.

quoted

The USB hardware will merge TCP receives and put multiple ethernet
packets into a single USB message.
But single frames can end up in very big kernel memory buffers.

Yeah, this is a known problem.

The whole USB ethernet block is somewhat horrid and inefficient
especially for XHCI/USB3 - which could have high speed ethernet.
It really needs to either directly interface to the XHCI ring
(like a normal ethernet driver) or be given the sequence of
USB rx packets to split/join into ethernet frames.

However I don't have the time to make those changes.
When I was looking at that driver 'dayjob' was actually
trying to make it work.
They dropped that idea later.
I've not got the ethernet dongle any more either.

	David

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