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.

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.

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(+)

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);

+/* pp_frag_count represents the number of writers who can update the page
+ * 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 mappings
+ * of the fragments.
+ *
+ * When pp_frag_count reaches 0 we can either recycle the page, if the page
+ * refcnt is 1, or return it back to the memory allocator and destroy any
+ * mappings we have.
+ */
 static inline void page_pool_fragment_page(struct page *page, long nr)
 {
 	atomic_long_set(&page->pp_frag_count, nr);
--
2.38.1

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.
> 
> 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.
> 
> 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(+)
> 
> 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);
> 
> +/* pp_frag_count represents the number of writers who can update the page
> + * 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 mappings
> + * of the fragments.
> + *
> + * When pp_frag_count reaches 0 we can either recycle the page, if the page
> + * refcnt is 1, or return it back to the memory allocator and destroy any
> + * 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>