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Message-Id: <20230127101627.891614-1-ilias.apalodimas@linaro.org>
Date: Fri, 27 Jan 2023 12:16:27 +0200
From: Ilias Apalodimas <ilias.apalodimas@...aro.org>
To: netdev@...r.kernel.org
Cc: alexander.duyck@...il.com,
Ilias Apalodimas <ilias.apalodimas@...aro.org>,
Jesper Dangaard Brouer <hawk@...nel.org>,
"David S. Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>, linux-kernel@...r.kernel.org
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.
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@...aro.org>
---
include/net/page_pool.h | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/include/net/page_pool.h b/include/net/page_pool.h
index 813c93499f20..115dbce6d431 100644
--- a/include/net/page_pool.h
+++ b/include/net/page_pool.h
@@ -277,6 +277,14 @@ void page_pool_put_defragged_page(struct page_pool *pool, struct page *page,
unsigned int dma_sync_size,
bool allow_direct);
+/* pp_frag_count is our number of outstanding DMA maps. 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);
--
2.38.1
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