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 <[email protected]>
---
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 <[email protected]>
> ---
> 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 <[email protected]>