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

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2023/3/3 19:22, Alexander Lobakin wrote:
> From: Yunsheng Lin <linyunsheng@huawei.com>
> 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@MW5PR15MB5121.namprd15.prod.outlook.com/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

The wireless adapter which use Page Pool *does* mix PP pages with
non-PP, see below discussion:

https://lore.kernel.org/netdev/156f3e120bd0757133cb6bc11b76889637b5e0a6.camel@gmail.com/

> 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@mail.gmail.com/
> 
> 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.

When we are freeing a page by updating the _refcount, I think
we are already touching the cache of ::pp_magic.

Anyway, I am not sure checking ::pp_magic is correct when a
page will be passing between different subsystem and back to
the network stack eventually, checking ::pp_magic may not be
correct if this happens.

Another way is to use the bottom two bits in bv_page, see:
https://www.spinics.net/lists/netdev/msg874099.html

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