linux-kernel - Re: [PATCH net-next v3 3/4] page_pool: introduce page_pool

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Message-ID: <0ba1bf9c-2e45-cd44-60d3-66feeb3268f3@redhat.com>
Date:   Thu, 15 Jun 2023 18:19:15 +0200
From:   Jesper Dangaard Brouer <jbrouer@...hat.com>
To:     Alexander Duyck <alexander.duyck@...il.com>,
        Yunsheng Lin <linyunsheng@...wei.com>
Cc:     brouer@...hat.com, davem@...emloft.net, kuba@...nel.org,
        pabeni@...hat.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Maryam Tahhan <mtahhan@...hat.com>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH net-next v3 3/4] page_pool: introduce page_pool_alloc()
 API

On 15/06/2023 16.45, Alexander Duyck wrote:
[..]
> 
> What concerns me is that you seem to be taking the page pool API in a
> direction other than what it was really intended for. For any physical
> device you aren't going to necessarily know what size fragment you are
> working with until you have already allocated the page and DMA has
> been performed. That is why drivers such as the Mellanox driver are
> fragmenting in the driver instead of allocated pre-fragmented pages.
> 

+1

I share concerns with Alexander Duyck here. As the inventor and
maintainer, I can say this is taking the page_pool API in a direction I
didn't intent or planned for. As Alex also says, the intent was for
fixed sized memory chunks that are DMA ready.  Use-case was the physical
device RX "early demux problem", where the size is not known before hand.

I need to be convinced this is a good direction to take the page_pool
design/architecture into... e.g. allocations with dynamic sizes.
Maybe it is a good idea, but as below "consumers" of the API is usually
the way to show this is the case.

[...]
> 
> What I was getting at is that if you are going to add an API you have
> to have a consumer for the API. That is rule #1 for kernel API
> development. You don't add API without a consumer for it. The changes
> you are making are to support some future implementation, and I see it
> breaking most of the existing implementation. That is my concern.
> 

You have mentioned veth as the use-case. I know I acked adding page_pool
use-case to veth, for when we need to convert an SKB into an
xdp_buff/xdp-frame, but maybe it was the wrong hammer(?).
In this case in veth, the size is known at the page allocation time.
Thus, using the page_pool API is wasting memory.  We did this for
performance reasons, but we are not using PP for what is was intended
for.  We mostly use page_pool, because it an existing recycle return
path, and we were too lazy to add another alloc-type (see enum
xdp_mem_type).

Maybe you/we can extend veth to use this dynamic size API, to show us
that this is API is a better approach.  I will signup for benchmarking
this (and coordinating with CC Maryam as she came with use-case we
improved on).

--Jesper