Re: [PATCH bpf-next v1 1/2] xdp: recycle Page Pool backed skbs built from XDP frames

[Alexander Lobakin]

From: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
Date: Thu, 2 Mar 2023 10:30:13 +0800

> On 2023/3/2 0:03, Alexander Lobakin wrote:
>> __xdp_build_skb_from_frame() state(d):
>>
>> /* Until page_pool get SKB return path, release DMA here */
>>
>> Page Pool got skb pages recycling in April 2021, but missed this
>> function.

[...]

> We both rely on both skb->pp_recycle and page->pp_magic to decide
> the page is really from page pool. So there was a few corner case
> problem when we are sharing a page for different skb in the driver
> level or calling skb_clone() or skb_try_coalesce().
> see:
> https://github.com/torvalds/linux/commit/2cc3aeb5ecccec0d266813172fcd82b4b5fa5803
> https://lore.kernel.org/netdev/MW5PR15MB51214C0513DB08A3607FBC1FBDE19@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/t/
> https://lore.kernel.org/netdev/167475990764.1934330.11960904198087757911.stgit@localhost.localdomain/

And they are fixed :D
No drivers currently which use Page Pool mix PP pages with non-PP. And
it's impossible to trigger try_coalesce() or so at least on cpumap path
since we're only creating skbs at that moment, they don't come from
anywhere else.

> 
> As the 'struct xdp_frame' also use 'struct skb_shared_info' which is
> sharable, see xdp_get_shared_info_from_frame().
> 
> For now xdpf_clone() does not seems to handling frag page yet,
> so it should be fine for now.

xdpf_clone() clones a frame to a new full page and doesn't copy its
skb_shared_info.

> 
> IMHO we should find a way to use per-page marker, instead of both
> per-skb and per-page markers, in order to avoid the above problem
> for xdp if xdp has a similar processing as skb, as suggested by Eric.
> 
> https://lore.kernel.org/netdev/CANn89iKgZU4Q+THXupzZi4hETuKuCOvOB=iHpp5JzQTNv_Fg_A@xxxxxxxxxxxxxx/

As Jesper already pointed out, not having a quick way to check whether
we have to check ::pp_magic at all can decrease performance. So it's
rather a shortcut.

> 
>>  
>>  	/* Allow SKB to reuse area used by xdp_frame */
>>  	xdp_scrub_frame(xdpf);
>>

Thanks,
Olek