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authorIlias Apalodimas <ilias.apalodimas@linaro.org>2023-02-18 00:21:30 +0200
committerJakub Kicinski <kuba@kernel.org>2023-02-21 09:15:39 -0800
commit4d4266e3fd321fadb628ce02de641b129522c39c (patch)
tree3a3b01a058c6b6deb48ef8f14630d1eae7bef589 /include/net
parenta00da30c052f07d67da56efd6a4f1fc85956c979 (diff)
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>
Diffstat (limited to 'include/net')
-rw-r--r--include/net/page_pool.h10
1 files changed, 10 insertions, 0 deletions
diff --git a/include/net/page_pool.h b/include/net/page_pool.h
index 34bf531ffc8d..ddfa0b328677 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);