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

[Ilias Apalodimas]

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.

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

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

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);
> > 
> > [...]
> 
>