linux-kernel - Re: [PATCH net-next v5 03/14] page_pool: avoid calling no-op externals when possible

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Date:   Wed, 29 Nov 2023 11:17:50 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     Alexander Lobakin <aleksander.lobakin@...el.com>,
        Christoph Hellwig <hch@....de>
CC:     Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Michal Kubiak <michal.kubiak@...el.com>,
        Larysa Zaremba <larysa.zaremba@...el.com>,
        Alexander Duyck <alexanderduyck@...com>,
        David Christensen <drc@...ux.vnet.ibm.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Paul Menzel <pmenzel@...gen.mpg.de>, <netdev@...r.kernel.org>,
        <intel-wired-lan@...ts.osuosl.org>, <linux-kernel@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net-next v5 03/14] page_pool: avoid calling no-op
 externals when possible

On 2023/11/27 22:32, Alexander Lobakin wrote:
> 
> Chris, any thoughts on a global flag for skipping DMA syncs ladder?

It seems there was one already in the past:

https://lore.kernel.org/netdev/7c55a4d7-b4aa-25d4-1917-f6f355bd722e@arm.com/T/

> 
>>
>>

>>> +static inline bool page_pool_set_dma_addr(const struct page_pool *pool,
>>> +					  struct page *page,
>>> +					  dma_addr_t addr)
>>>  {
>>> +	unsigned long val = addr;
>>> +
>>> +	if (unlikely(!addr)) {
>>> +		page->dma_addr = 0;
>>> +		return true;
>>> +	}
>>
>> The above seems unrelated change?
> 
> Related. We use page_put_set_dma_addr() to clear ::dma_addr as well
> (grep for it in page_pool.c). In this case, we don't want
> dma_need_sync() to be called as we explicitly pass zero. This check
> zeroes the field and exits as quickly as possible.

The question seems to be about if we need to ensure the LSB of
page->dma_addr is not set when page_pool releases a page back to page
allocator?

> In case with the call mentioned above, zero is a compile-time constant
> there, so that this little branch will be inlined with the rest dropped.
>