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

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2021/5/14 17:17, Ilias Apalodimas wrote:
> On Fri, May 14, 2021 at 04:31:50PM +0800, Yunsheng Lin wrote:
>> 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.
>>
> 
> The driver is meant to call skb_mark_for_recycle for the skb and
> page_pool_store_mem_info() for the fragments (in order to store page->pp).
> Nothing bad will happen if you call skb_mark_for_recycle on a frag though,
> but in any case you need to store the page_pool pointer of each frag to
> struct page.

Right. Nothing bad will happen when we keep the page_pool pointer in
page->pp while page pool owns the page too, even if the skb->pp_recycle
is not set, right?

> 
>> 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?
> 
> Then you'd have to check the page pool config before saving the meta-data,

I am not sure what the "saving the meta-data" meant?

> and you would have to make the skb path aware of that as well (I assume you
> mean replace pp_recycle with this?).

I meant we could set the in page->pp when the page is allocated from
alloc_pages() in __page_pool_alloc_pages_slow() unconditionally or
according to a newly add filed in pool->p, and only clear it in
page_pool_release_page(), between which the page is owned by page pool,
right?

> If not and you just want to add an extra flag on page_pool_params and be able 
> to enable recycling depending on that flag, we just add a patch afterwards.
> I am not sure we need an extra if for each packet though.

In that case, the skb_mark_for_recycle() could only set the skb->pp_recycle,
but not the pool->p.

> 
>>
>> 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?
>>
> 
> Not really, the driver has 2 choices:
> - call page_pool_release_page() once it receives the payload. That will
>   clean up dma mappings (if page pool is responsible for them) and free the
>   buffer

The is only needed before SKB recycling is supported or the driver does not
want the SKB recycling support explicitly, right?

> - call skb_mark_for_recycle(). Which will end up recycling the buffer.

If the driver need to add extra flag to enable recycling based on skb
instead of page pool, then adding skb_mark_for_recycle() makes sense to
me too, otherwise it seems adding a field in pool->p to recycling based
on skb makes more sense?

> 
> If you call none of those, you'd leak a page, but that's a driver bug.
> patches [4/5, 5/5] do that for two marvell drivers.
> I really want to make drivers opt-in in the feature instead of always
> enabling it.
> 
> Thanks
> /Ilias
>>
>>>
>>>>> +	page_pool_put_full_page(pp, virt_to_head_page(data), false);
>>>>> +
>>>>>  	C(end);
>>>
>>> [...]
>>
>>
> 
> .
>