Re: [RFC] net: esp: fix bad handling of pages from page_pool

[Dragos Tatulea <dtatulea@nvidia.com>]

On Wed, 2024-03-06 at 08:40 -0800, Mina Almasry wrote:
> On Wed, Mar 6, 2024 at 8:09 AM Jakub Kicinski <kuba@kernel.org> wrote:
> > 
> > On Wed, 6 Mar 2024 16:00:46 +0000 Dragos Tatulea wrote:
> > > > Hm, that's a judgment call.
> > > > Part of me wants to put it next to napi_frag_unref(), since we
> > > > basically need to factor out the insides of this function.
> > > > When you post the patch the page pool crowd will give us
> > > > their opinions.
> > > 
> > > Why not have napi_pp_put_page simply return false if CONFIG_PAGE_POOL is not
> > > set?
> > 
> > Without LTO it may still be a function call.
> > Plus, subjectively, I think that it's a bit too much logic to encode in
> > the caller (you must also check skb->pp_recycle, AFAIU)
> > Maybe we should make skb_pp_recycle() take struct page and move it to
> > skbuff.h ? Rename it to skb_page_unref() ?
> > 
> 
> Does the caller need to check skb->pp_recycle? pp_recycle seems like a
> redundant bit. We can tell whether the page is pp by checking
> is_pp_page(page). the pages in the frag must be pp pages when
> skb->pp_recycle is set and must be non pp pages when the
> skb->pp_recycle is not set, so it all seems redundant to me.
> 
AFAIU we don't have to check for pp_recycle, at least not in this specific case.

> My fix would be something like:
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index d577e0bee18d..cc737b7b9860 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -3507,17 +3507,25 @@ int skb_cow_data_for_xdp(struct page_pool
> *pool, struct sk_buff **pskb,
>  bool napi_pp_put_page(struct page *page, bool napi_safe);
> 
>  static inline void
> -napi_frag_unref(skb_frag_t *frag, bool recycle, bool napi_safe)
> +napi_page_unref(struct page *page, bool napi_safe)
>  {
> -       struct page *page = skb_frag_page(frag);
> -
>  #ifdef CONFIG_PAGE_POOL
> -       if (recycle && napi_pp_put_page(page, napi_safe))
> +       if (napi_pp_put_page(page, napi_safe))
>                 return;
>  #endif
>         put_page(page);
>  }
> 
> +static inline void
> +napi_frag_unref(skb_frag_t *frag, bool recycle, bool napi_safe)
> +{
> +       struct page *page = skb_frag_page(frag);
> +
> +       DEBUG_NET_WARN_ON(recycle != is_pp_page(page));
> +
> +       napi_page_unref(page);
> +}
> +
> 
> And then use napi_page_unref() in the callers to handle page pool &
> non-page pool gracefully without leaking page pool internals to the
> callers.
> 
We'd also need to add is_pp_page() in the header with the changes above...

On that line of thought, unless these new APIs are useful for other use-cases,
why not keep it simple:
- Move is_pp_page() to skbuff.h.
- Do a simple is_pp_page(page) ? page_pool_put_full_page(page):put_page(page) in
the caller? Checking skb->pp_recycle would not be needed. 

Thanks,
Dragos

> > > Regarding stable would I need to send a separate fix that does the raw pp page
> > > check without the API?
> > 
> > You can put them in one patch, I reckon.
>