netdev - Re: [RFC PATCH net-next v1 4/4] net: page_pool: use netmem

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Date: Thu, 14 Dec 2023 08:27:55 -0800
From: Mina Almasry <almasrymina@...gle.com>
To: Yunsheng Lin <linyunsheng@...wei.com>
Cc: linux-kernel@...r.kernel.org, netdev@...r.kernel.org, bpf@...r.kernel.org, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
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	Willem de Bruijn <willemdebruijn.kernel@...il.com>
Subject: Re: [RFC PATCH net-next v1 4/4] net: page_pool: use netmem_t instead
 of struct page in API

On Thu, Dec 14, 2023 at 4:05 AM Yunsheng Lin <linyunsheng@...wei.com> wrote:
>
> On 2023/12/14 10:05, Mina Almasry wrote:
>
> ...
>
> > diff --git a/include/net/page_pool/types.h b/include/net/page_pool/types.h
> > index ac286ea8ce2d..0faa5207a394 100644
> > --- a/include/net/page_pool/types.h
> > +++ b/include/net/page_pool/types.h
> > @@ -6,6 +6,7 @@
> >  #include <linux/dma-direction.h>
> >  #include <linux/ptr_ring.h>
> >  #include <linux/types.h>
> > +#include <net/netmem.h>
> >
> >  #define PP_FLAG_DMA_MAP              BIT(0) /* Should page_pool do the DMA
> >                                       * map/unmap
> > @@ -199,9 +200,9 @@ struct page_pool {
> >       } user;
> >  };
> >
> > -struct page *page_pool_alloc_pages(struct page_pool *pool, gfp_t gfp);
> > -struct page *page_pool_alloc_frag(struct page_pool *pool, unsigned int *offset,
> > -                               unsigned int size, gfp_t gfp);
> > +struct netmem *page_pool_alloc_pages(struct page_pool *pool, gfp_t gfp);
> > +struct netmem *page_pool_alloc_frag(struct page_pool *pool, unsigned int *offset,
> > +                            unsigned int size, gfp_t gfp);
>
> Is it possible that we add a thin layer caller on top of the page_pool API?
> So that the existing users can still use the old API, the new user supporting
> the devmem can use the new API, something like below:
>
> struct netmem *netmem_pool_alloc(struct netmem_pool *pool, gfp_t gfp)
> or
> struct devmem *devmem_pool_alloc(struct devmem_pool *pool, gfp_t gfp)
>

Yes, it can be a thin layer on top of the page_pool API, retaining the
support for the old API, so that I don't have to modify existing
users. But I have to tweak it slightly, it would be something like:

struct page *page_pool_alloc_pages(struct page_pool *pool, gfp_t gfp);
/* old api unchanged */
+struct netmem *page_pool_alloc_netmem(struct page_pool *pool, gfp_t
gfp); /* new api added */

The new API can be implemented like this, but I don't need to add it
right now, it can be added in the separate devmem series:

struct netmem *page_pool_alloc_netmem(struct page_pool *pool, gfp_t gfp)
{
    return page_to_netmem(page_pool_alloc_pages(pool, gfp);
}

Willem I think suggested something similar to this. Drivers can then
use the old API while we implement the new API that supports different
memory types.

To support drivers using the old API I need to add a new skb frag
helper rather than modify the existing one:

+ void skb_add_rx_frag_netmem(struct sk_buff *skb, int i, struct
netmem *netmem, int off,
+                                                     int size,
unsigned int truesize)

> I perfer the second one personally, as devmem means that it is not
> readable from cpu.

>From my POV it has to be the first one. We want to abstract the memory
type from the drivers as much as possible, not introduce N new memory
types and ask the driver to implement new code for each of them
separately.

> Perhaps netmem can be used in the networking core in the future to
> indicate the generic type for all types of memory supported by networking
> core.
>
> As the main concern from Jason seems to be about safe type protection for
> large driver facing API surface. And touching a lot of existing users does
> not seem to bring a lot of benefit when we have not a clear idea how to
> proceed yet.



-- 
Thanks,
Mina