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Date:   Mon, 6 Nov 2023 13:02:39 -0800
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Subject: Re: [RFC PATCH v3 06/12] memory-provider: dmabuf devmem memory provider
From:   Stanislav Fomichev <sdf@google.com>
To:     Mina Almasry <almasrymina@google.com>
Cc:     netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-arch@vger.kernel.org, linux-kselftest@vger.kernel.org,
        linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org,
        linaro-mm-sig@lists.linaro.org,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        Ilias Apalodimas <ilias.apalodimas@linaro.org>,
        Arnd Bergmann <arnd@arndb.de>,
        David Ahern <dsahern@kernel.org>,
        Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
        Shuah Khan <shuah@kernel.org>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        "Christian =?utf-8?B?S8O2bmln?=" <christian.koenig@amd.com>,
        Shakeel Butt <shakeelb@google.com>,
        Jeroen de Borst <jeroendb@google.com>,
        Praveen Kaligineedi <pkaligineedi@google.com>,
        Willem de Bruijn <willemb@google.com>,
        Kaiyuan Zhang <kaiyuanz@google.com>
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On 11/05, Mina Almasry wrote:
> Implement a memory provider that allocates dmabuf devmem page_pool_iovs.
> 
> Support of PP_FLAG_DMA_MAP and PP_FLAG_DMA_SYNC_DEV is omitted for
> simplicity.
> 
> The provider receives a reference to the struct netdev_dmabuf_binding
> via the pool->mp_priv pointer. The driver needs to set this pointer for
> the provider in the page_pool_params.
> 
> The provider obtains a reference on the netdev_dmabuf_binding which
> guarantees the binding and the underlying mapping remains alive until
> the provider is destroyed.
> 
> Signed-off-by: Willem de Bruijn <willemb@google.com>
> Signed-off-by: Kaiyuan Zhang <kaiyuanz@google.com>
> Signed-off-by: Mina Almasry <almasrymina@google.com>
> 
> ---
>  include/net/page_pool/helpers.h | 40 +++++++++++++++++
>  include/net/page_pool/types.h   | 10 +++++
>  net/core/page_pool.c            | 76 +++++++++++++++++++++++++++++++++
>  3 files changed, 126 insertions(+)
> 
> diff --git a/include/net/page_pool/helpers.h b/include/net/page_pool/helpers.h
> index 78cbb040af94..b93243c2a640 100644
> --- a/include/net/page_pool/helpers.h
> +++ b/include/net/page_pool/helpers.h
> @@ -53,6 +53,7 @@
>  #define _NET_PAGE_POOL_HELPERS_H
>  
>  #include <net/page_pool/types.h>
> +#include <net/net_debug.h>
>  
>  #ifdef CONFIG_PAGE_POOL_STATS
>  int page_pool_ethtool_stats_get_count(void);
> @@ -111,6 +112,45 @@ page_pool_iov_binding(const struct page_pool_iov *ppiov)
>  	return page_pool_iov_owner(ppiov)->binding;
>  }
>  
> +static inline int page_pool_iov_refcount(const struct page_pool_iov *ppiov)
> +{
> +	return refcount_read(&ppiov->refcount);
> +}
> +
> +static inline void page_pool_iov_get_many(struct page_pool_iov *ppiov,
> +					  unsigned int count)
> +{
> +	refcount_add(count, &ppiov->refcount);
> +}
> +
> +void __page_pool_iov_free(struct page_pool_iov *ppiov);
> +
> +static inline void page_pool_iov_put_many(struct page_pool_iov *ppiov,
> +					  unsigned int count)
> +{
> +	if (!refcount_sub_and_test(count, &ppiov->refcount))
> +		return;
> +
> +	__page_pool_iov_free(ppiov);
> +}
> +
> +/* page pool mm helpers */
> +
> +static inline bool page_is_page_pool_iov(const struct page *page)
> +{
> +	return (unsigned long)page & PP_DEVMEM;
> +}

Speaking of bpf: one thing that might be problematic with this PP_DEVMEM
bit is that it will make debugging with bpftrace a bit (more)
complicated. If somebody were trying to get to that page_pool_iov from
the frags, they will have to do the equivalent of page_is_page_pool_iov,
but probably not a big deal? (thinking out loud)