netdev - Re: [PATCH net-next v5 3/5] page_pool: Allow drivers to hint on SKB recycling

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Date:   Fri, 14 May 2021 16:31:50 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     Ilias Apalodimas <ilias.apalodimas@...aro.org>
CC:     Matteo Croce <mcroce@...ux.microsoft.com>,
        <netdev@...r.kernel.org>, <linux-mm@...ck.org>,
        Ayush Sawal <ayush.sawal@...lsio.com>,
        "Vinay Kumar Yadav" <vinay.yadav@...lsio.com>,
        Rohit Maheshwari <rohitm@...lsio.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Marcin Wojtas <mw@...ihalf.com>,
        Russell King <linux@...linux.org.uk>,
        Mirko Lindner <mlindner@...vell.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        "Tariq Toukan" <tariqt@...dia.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        "Alexei Starovoitov" <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "John Fastabend" <john.fastabend@...il.com>,
        Boris Pismenny <borisp@...dia.com>,
        Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>, Yu Zhao <yuzhao@...gle.com>,
        Will Deacon <will@...nel.org>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Roman Gushchin <guro@...com>, Hugh Dickins <hughd@...gle.com>,
        Peter Xu <peterx@...hat.com>, Jason Gunthorpe <jgg@...pe.ca>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Alexander Lobakin <alobakin@...me>,
        Cong Wang <cong.wang@...edance.com>, wenxu <wenxu@...oud.cn>,
        Kevin Hao <haokexin@...il.com>,
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        Marco Elver <elver@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Guillaume Nault <gnault@...hat.com>,
        <linux-kernel@...r.kernel.org>, <linux-rdma@...r.kernel.org>,
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        Eric Dumazet <edumazet@...gle.com>,
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        Saeed Mahameed <saeedm@...dia.com>,
        Andrew Lunn <andrew@...n.ch>, Paolo Abeni <pabeni@...hat.com>,
        Sven Auhagen <sven.auhagen@...eatech.de>
Subject: Re: [PATCH net-next v5 3/5] page_pool: Allow drivers to hint on SKB
 recycling

On 2021/5/14 15:36, Ilias Apalodimas wrote:
> [...]
>>> +		return false;
>>> +
>>> +	pp = (struct page_pool *)page->pp;
>>> +
>>> +	/* Driver set this to memory recycling info. Reset it on recycle.
>>> +	 * This will *not* work for NIC using a split-page memory model.
>>> +	 * The page will be returned to the pool here regardless of the
>>> +	 * 'flipped' fragment being in use or not.
>>> +	 */
>>> +	page->pp = NULL;
>>
>> Why not only clear the page->pp when the page can not be recycled
>> by the page pool? so that we do not need to set and clear it every
>> time the page is recycled。
>>
> 
> If the page cannot be recycled, page->pp will not probably be set to begin
> with. Since we don't embed the feature in page_pool and we require the
> driver to explicitly enable it, as part of the 'skb flow', I'd rather keep 
> it as is.  When we set/clear the page->pp, the page is probably already in 
> cache, so I doubt this will have any measurable impact.

The point is that we already have the skb->pp_recycle to let driver to
explicitly enable recycling, as part of the 'skb flow, if the page pool keep
the page->pp while it owns the page, then the driver may only need to call
one skb_mark_for_recycle() for a skb, instead of call skb_mark_for_recycle()
for each page frag of a skb.

Maybe we can add a parameter in "struct page_pool_params" to let driver
to decide if the page pool ptr is stored in page->pp while the page pool
owns the page?

Another thing accured to me is that if the driver use page from the
page pool to form a skb, and it does not call skb_mark_for_recycle(),
then there will be resource leaking, right? if yes, it seems the
skb_mark_for_recycle() call does not seems to add any value?


> 
>>> +	page_pool_put_full_page(pp, virt_to_head_page(data), false);
>>> +
>>>  	C(end);
> 
> [...]