To eliminate the use of struct page in page pool, the page pool users
should use netmem descriptor and APIs instead.

Make mlx4 access ->pp_ref_count through netmem_desc instead of page.

While at it, add a helper, pp_page_to_nmdesc() and __pp_page_to_nmdesc(),
that can be used to get netmem_desc from page only if it's a pp page.
For now that netmem_desc overlays on page, it can be achieved by just
casting, and use macro and _Generic to cover const casting as well.

Plus, change page_pool_page_is_pp() to check for 'const struct page *'
instead of 'struct page *' since it doesn't modify data and additionally
covers const type.

Signed-off-by: Byungchul Park <byungchul@sk.com>
---
 drivers/net/ethernet/mellanox/mlx4/en_rx.c |  4 +++-
 include/linux/mm.h                         |  4 ++--
 include/net/netmem.h                       | 17 +++++++++++++++++
 3 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/mellanox/mlx4/en_rx.c b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
index b33285d755b9..92a16ddb7d86 100644
--- a/drivers/net/ethernet/mellanox/mlx4/en_rx.c
+++ b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
@@ -460,9 +460,11 @@ static int mlx4_en_complete_rx_desc(struct mlx4_en_priv *priv,
 
 		truesize += frag_info->frag_stride;
 		if (frag_info->frag_stride == PAGE_SIZE / 2) {
+			struct netmem_desc *desc = pp_page_to_nmdesc(page);
+
 			frags->page_offset ^= PAGE_SIZE / 2;
 			release = page_count(page) != 1 ||
-				  atomic_long_read(&page->pp_ref_count) != 1 ||
+				  atomic_long_read(&desc->pp_ref_count) != 1 ||
 				  page_is_pfmemalloc(page) ||
 				  page_to_nid(page) != numa_mem_id();
 		} else if (!priv->rx_headroom) {
diff --git a/include/linux/mm.h b/include/linux/mm.h
index fa538feaa8d9..ae50c1641bed 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -4178,12 +4178,12 @@ int arch_lock_shadow_stack_status(struct task_struct *t, unsigned long status);
 #define PP_MAGIC_MASK ~(PP_DMA_INDEX_MASK | 0x3UL)
 
 #ifdef CONFIG_PAGE_POOL
-static inline bool page_pool_page_is_pp(struct page *page)
+static inline bool page_pool_page_is_pp(const struct page *page)
 {
 	return (page->pp_magic & PP_MAGIC_MASK) == PP_SIGNATURE;
 }
 #else
-static inline bool page_pool_page_is_pp(struct page *page)
+static inline bool page_pool_page_is_pp(const struct page *page)
 {
 	return false;
 }
diff --git a/include/net/netmem.h b/include/net/netmem.h
index 097bc74d9555..f7dacc9e75fd 100644
--- a/include/net/netmem.h
+++ b/include/net/netmem.h
@@ -285,6 +285,23 @@ static inline struct net_iov *__netmem_clear_lsb(netmem_ref netmem)
 	return (struct net_iov *)((__force unsigned long)netmem & ~NET_IOV);
 }
 
+/* XXX: How to extract netmem_desc from page must be changed, once
+ * netmem_desc no longer overlays on page and will be allocated through
+ * slab.
+ */
+#define __pp_page_to_nmdesc(p)	(_Generic((p),				\
+	const struct page * :	(const struct netmem_desc *)(p),	\
+	struct page * :		(struct netmem_desc *)(p)))
+
+/* CAUTION: Check if the page is a pp page before calling this helper or
+ * know it's a pp page.
+ */
+#define pp_page_to_nmdesc(p)						\
+({									\
+	DEBUG_NET_WARN_ON_ONCE(!page_pool_page_is_pp(p));		\
+	__pp_page_to_nmdesc(p);						\
+})
+
 /**
  * __netmem_get_pp - unsafely get pointer to the &page_pool backing @netmem
  * @netmem: netmem reference to get the pointer from
-- 
2.17.1