Message-ID: <3bc180079f301dcbc24596b826c94c56424ba6c8.camel@gmail.com>
Subject: Re: [PATCH v2] page_pool: add a comment explaining the fragment
 counter usage
From:   Alexander H Duyck <alexander.duyck@gmail.com>
To:     Ilias Apalodimas <ilias.apalodimas@linaro.org>,
        netdev@vger.kernel.org
Cc:     Jesper Dangaard Brouer <hawk@kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>, linux-kernel@vger.kernel.org
Date:   Tue, 14 Feb 2023 07:27:41 -0800
In-Reply-To: <20230214104316.2254814-1-ilias.apalodimas@linaro.org>
References: <20230214104316.2254814-1-ilias.apalodimas@linaro.org>
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On Tue, 2023-02-14 at 12:43 +0200, Ilias Apalodimas wrote:
> When reading the page_pool code the first impression is that keeping
> two separate counters, one being the page refcnt and the other being
> fragment pp_frag_count, is counter-intuitive.
>=20
> However without that fragment counter we don't know when to reliably
> destroy or sync the outstanding DMA mappings.  So let's add a comment
> explaining this part.
>=20
> Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
> ---
> Changes since v1:
> - Update the comment withe the correct description for pp_frag_count
>  include/net/page_pool.h | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>=20
> diff --git a/include/net/page_pool.h b/include/net/page_pool.h
> index 34bf531ffc8d..277e215cfb58 100644
> --- a/include/net/page_pool.h
> +++ b/include/net/page_pool.h
> @@ -277,6 +277,16 @@ void page_pool_put_defragged_page(struct page_pool *=
pool, struct page *page,
>  				  unsigned int dma_sync_size,
>  				  bool allow_direct);
>=20
> +/* pp_frag_count represents the number of writers who can update the pag=
e
> + * either by updating skb->data or via DMA mappings for the device.
> + * We can't rely on the page refcnt, as we don't know who might be
> + * holding page references and we can't reliably destroy or sync DMA map=
pings
> + * of the fragments.
> + *
> + * When pp_frag_count reaches 0 we can either recycle the page, if the p=
age
> + * refcnt is 1, or return it back to the memory allocator and destroy an=
y
> + * mappings we have.
> + */

I would get rid of the comma between "page" and "if" in the second
paragraph. It breaks things up and makes it a bit harder to read. What
we want to emphasize is that there are two possible paths. The extra
comma makes it almost appear as if there are 3 options.

Otherwise it looks good to me.

Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>