From 4d4266e3fd321fadb628ce02de641b129522c39c Mon Sep 17 00:00:00 2001
From: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Date: Sat, 18 Feb 2023 00:21:30 +0200
Subject: [PATCH] page_pool: add a comment explaining the fragment counter
usage
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.
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Link: https://lore.kernel.org/r/20230217222130.85205-1-ilias.apalodimas@linaro.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
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 34bf531ffc8d6..ddfa0b3286777 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 for that 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);
--
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