Re: [PATCH net-next v12 06/13] page_pool: devmem support

[Pavel Begunkov <asml.silence@xxxxxxxxx>]

On 6/13/24 02:35, Mina Almasry wrote:
> Convert netmem to be a union of struct page and struct netmem. Overload
> the LSB of struct netmem* to indicate that it's a net_iov, otherwise
> it's a page.
>
> Currently these entries in struct page are rented by the page_pool and
> used exclusively by the net stack:
>
> struct {
> 	unsigned long pp_magic;
> 	struct page_pool *pp;
> 	unsigned long _pp_mapping_pad;
> 	unsigned long dma_addr;
> 	atomic_long_t pp_ref_count;
> };
>
> Mirror these (and only these) entries into struct net_iov and implement
> netmem helpers that can access these common fields regardless of
> whether the underlying type is page or net_iov.
>
> Implement checks for net_iov in netmem helpers which delegate to mm
> APIs, to ensure net_iov are never passed to the mm stack.
>
> Signed-off-by: Mina Almasry <almasrymina@xxxxxxxxxx>

Apart from small comments below

Reviewed-by: Pavel Begunkov <asml.silence@xxxxxxxxx>


> ---
>   include/net/netmem.h            | 137 ++++++++++++++++++++++++++++++--
>   include/net/page_pool/helpers.h |  25 +++---
>   net/core/devmem.c               |   3 +
>   net/core/page_pool.c            |  26 +++---
>   net/core/skbuff.c               |  22 +++--
>   5 files changed, 168 insertions(+), 45 deletions(-)
>
> diff --git a/include/net/netmem.h b/include/net/netmem.h
> index 664df8325ece5..35ad237fdf29e 100644
> --- a/include/net/netmem.h
> +++ b/include/net/netmem.h
...
> -/* Converting from page to netmem is always safe, because a page can always be
> - * a netmem.
> - */
>   static inline netmem_ref page_to_netmem(struct page *page)
>   {
>   	return (__force netmem_ref)page;
> @@ -68,17 +107,103 @@ static inline netmem_ref page_to_netmem(struct page *page)
>   
>   static inline int netmem_ref_count(netmem_ref netmem)
>   {
> +	/* The non-pp refcount of net_iov is always 1. On net_iov, we only
> +	 * support pp refcounting which uses the pp_ref_count field.
> +	 */
> +	if (netmem_is_net_iov(netmem))
> +		return 1;
> +
>   	return page_ref_count(netmem_to_page(netmem));
>   }
>   
>   static inline unsigned long netmem_to_pfn(netmem_ref netmem)
>   {
> +	if (netmem_is_net_iov(netmem))
> +		return 0;

IIRC 0 is a valid pfn. Not much of a concern since it's
used only for tracing, but might make sense to pass some
invalid pfn if there is one

> +
>   	return page_to_pfn(netmem_to_page(netmem));
>   }
>   
...
>   static inline netmem_ref netmem_compound_head(netmem_ref netmem)
>   {
> +	/* niov are never compounded */
> +	if (netmem_is_net_iov(netmem))
> +		return netmem;
> +
>   	return page_to_netmem(compound_head(netmem_to_page(netmem)));
>   }
>   
> +static inline void *netmem_address(netmem_ref netmem)

I don't think it's used anywhere, do I miss it?

> +{
> +	if (netmem_is_net_iov(netmem))
> +		return NULL;
> +
> +	return page_address(netmem_to_page(netmem));
> +}
> +
...
> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> index a5957d3359762..1152e3547795a 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -26,6 +26,8 @@
...
>   
>   /* If the page refcnt == 1, this will try to recycle the page.
> @@ -714,7 +713,7 @@ __page_pool_put_page(struct page_pool *pool, netmem_ref netmem,
>   	 * refcnt == 1 means page_pool owns page, and can recycle it.
>   	 *
>   	 * page is NOT reusable when allocated when system is under
> -	 * some pressure. (page_is_pfmemalloc)
> +	 * some pressure. (page_pool_page_is_pfmemalloc)

There is no page_pool_page_is_pfmemalloc()

>   	 */
>   	if (likely(__page_pool_page_can_be_recycled(netmem))) {
>   		/* Read barrier done in page_ref_count / READ_ONCE */
> @@ -727,6 +726,7 @@ __page_pool_put_page(struct page_pool *pool, netmem_ref netmem,
>   		/* Page found as candidate for recycling */
>   		return netmem;
>   	}

--
Pavel Begunkov