[PATCH v2] page_pool: add a comment explaining the fragment counter usage

[Ilias Apalodimas]

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@xxxxxxxxxx>
---
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