[PATCH net-next 1/2] page_pool: unify frag page and non-frag page handling

[Yunsheng Lin]

Currently page_pool_dev_alloc_pages() can not be called
when PP_FLAG_PAGE_FRAG is set, because it does not use
the frag reference counting.

As we are already doing a optimization by not updating
page->pp_frag_count in page_pool_defrag_page() for the
last frag user, and non-frag page only have one user,
so we utilize that to unify frag page and non-frag page
handling, so that page_pool_dev_alloc_pages() can also
be called with PP_FLAG_PAGE_FRAG set.

Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
CC: Lorenzo Bianconi <lorenzo@xxxxxxxxxx>
CC: Alexander Duyck <alexander.duyck@xxxxxxxxx>
---
 include/net/page_pool.h | 38 +++++++++++++++++++++++++++++++-------
 net/core/page_pool.c    |  1 +
 2 files changed, 32 insertions(+), 7 deletions(-)

diff --git a/include/net/page_pool.h b/include/net/page_pool.h
index c8ec2f34722b..ea7a0c0592a5 100644
--- a/include/net/page_pool.h
+++ b/include/net/page_pool.h
@@ -50,6 +50,9 @@
 				 PP_FLAG_DMA_SYNC_DEV |\
 				 PP_FLAG_PAGE_FRAG)
 
+#define PAGE_POOL_DMA_USE_PP_FRAG_COUNT \
+		(sizeof(dma_addr_t) > sizeof(unsigned long))
+
 /*
  * Fast allocation side cache array/stack
  *
@@ -295,13 +298,20 @@ void page_pool_put_defragged_page(struct page_pool *pool, struct page *page,
  */
 static inline void page_pool_fragment_page(struct page *page, long nr)
 {
-	atomic_long_set(&page->pp_frag_count, nr);
+	if (!PAGE_POOL_DMA_USE_PP_FRAG_COUNT)
+		atomic_long_set(&page->pp_frag_count, nr);
 }
 
+/* We need to reset frag_count back to 1 for the last user to allow
+ * only one user in case the page is recycled and allocated as non-frag
+ * page.
+ */
 static inline long page_pool_defrag_page(struct page *page, long nr)
 {
 	long ret;
 
+	BUILD_BUG_ON(__builtin_constant_p(nr) && nr != 1);
+
 	/* If nr == pp_frag_count then we have cleared all remaining
 	 * references to the page. No need to actually overwrite it, instead
 	 * we can leave this to be overwritten by the calling function.
@@ -311,19 +321,36 @@ static inline long page_pool_defrag_page(struct page *page, long nr)
 	 * especially when dealing with a page that may be partitioned
 	 * into only 2 or 3 pieces.
 	 */
-	if (atomic_long_read(&page->pp_frag_count) == nr)
+	if (atomic_long_read(&page->pp_frag_count) == nr) {
+		/* As we have ensured nr is always one for constant case
+		 * using the BUILD_BUG_ON() as above, only need to handle
+		 * the non-constant case here for frag count draining.
+		 */
+		if (!__builtin_constant_p(nr))
+			atomic_long_set(&page->pp_frag_count, 1);
+
 		return 0;
+	}
 
 	ret = atomic_long_sub_return(nr, &page->pp_frag_count);
 	WARN_ON(ret < 0);
+
+	/* Reset frag count back to 1, this should be the rare case when
+	 * two users call page_pool_defrag_page() currently.
+	 */
+	if (!ret)
+		atomic_long_set(&page->pp_frag_count, 1);
+
 	return ret;
 }
 
 static inline bool page_pool_is_last_frag(struct page_pool *pool,
 					  struct page *page)
 {
-	/* If fragments aren't enabled or count is 0 we were the last user */
-	return !(pool->p.flags & PP_FLAG_PAGE_FRAG) ||
+	/* When dma_addr_upper is overlapped with pp_frag_count
+	 * or we were the last page frag user.
+	 */
+	return PAGE_POOL_DMA_USE_PP_FRAG_COUNT ||
 	       (page_pool_defrag_page(page, 1) == 0);
 }
 
@@ -357,9 +384,6 @@ static inline void page_pool_recycle_direct(struct page_pool *pool,
 	page_pool_put_full_page(pool, page, true);
 }
 
-#define PAGE_POOL_DMA_USE_PP_FRAG_COUNT	\
-		(sizeof(dma_addr_t) > sizeof(unsigned long))
-
 static inline dma_addr_t page_pool_get_dma_addr(struct page *page)
 {
 	dma_addr_t ret = page->dma_addr;
diff --git a/net/core/page_pool.c b/net/core/page_pool.c
index e212e9d7edcb..0868aa8f6323 100644
--- a/net/core/page_pool.c
+++ b/net/core/page_pool.c
@@ -334,6 +334,7 @@ static void page_pool_set_pp_info(struct page_pool *pool,
 {
 	page->pp = pool;
 	page->pp_magic |= PP_SIGNATURE;
+	page_pool_fragment_page(page, 1);
 	if (pool->p.init_callback)
 		pool->p.init_callback(page, pool->p.init_arg);
 }
-- 
2.33.0