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

However, some code e.g. __netmem_to_page() is still used to access the
page pool fields e.g. ->pp via struct page, which should be changed so
as to access them via netmem descriptor, struct netmem_desc instead,
since the fields no longer will be available in struct page.

Introduce utility APIs to make them easy to use struct netmem_desc as
descriptor.  The APIs are:

   1. __netmem_to_nmdesc(), to convert netmem_ref to struct netmem_desc,
      but unsafely without checking if it's net_iov or system memory.

   2. netmem_to_nmdesc(), to convert netmem_ref to struct netmem_desc,
      safely with checking if it's net_iov or system memory.

   3. nmdesc_to_page(), to convert struct netmem_desc to struct page,
      assuming struct netmem_desc overlays on struct page.

   4. page_to_nmdesc(), to convert struct page to struct netmem_desc,
      assuming struct netmem_desc overlays on struct page, allowing only
      head page to be converted.

   5. nmdesc_adress(), to get its virtual address corresponding to the
      struct netmem_desc.

Signed-off-by: Byungchul Park <byungchul@sk.com>
---
 include/net/netmem.h | 41 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 41 insertions(+)

diff --git a/include/net/netmem.h b/include/net/netmem.h
index 535cf17b9134..ad9444be229a 100644
--- a/include/net/netmem.h
+++ b/include/net/netmem.h
@@ -198,6 +198,32 @@ static inline struct page *netmem_to_page(netmem_ref netmem)
 	return __netmem_to_page(netmem);
 }
 
+/**
+ * __netmem_to_nmdesc - unsafely get pointer to the &netmem_desc backing
+ * @netmem
+ * @netmem: netmem reference to convert
+ *
+ * Unsafe version of netmem_to_nmdesc(). When @netmem is always backed
+ * by system memory, performs faster and generates smaller object code
+ * (no check for the LSB, no WARN). When @netmem points to IOV, provokes
+ * undefined behaviour.
+ *
+ * Return: pointer to the &netmem_desc (garbage if @netmem is not backed
+ * by system memory).
+ */
+static inline struct netmem_desc *__netmem_to_nmdesc(netmem_ref netmem)
+{
+	return (__force struct netmem_desc *)netmem;
+}
+
+static inline struct netmem_desc *netmem_to_nmdesc(netmem_ref netmem)
+{
+	if (WARN_ON_ONCE(netmem_is_net_iov(netmem)))
+		return NULL;
+
+	return __netmem_to_nmdesc(netmem);
+}
+
 static inline struct net_iov *netmem_to_net_iov(netmem_ref netmem)
 {
 	if (netmem_is_net_iov(netmem))
@@ -314,6 +340,21 @@ static inline netmem_ref netmem_compound_head(netmem_ref netmem)
 	return page_to_netmem(compound_head(netmem_to_page(netmem)));
 }
 
+#define nmdesc_to_page(nmdesc)		(_Generic((nmdesc),		\
+	const struct netmem_desc * :	(const struct page *)(nmdesc),	\
+	struct netmem_desc * :		(struct page *)(nmdesc)))
+
+static inline struct netmem_desc *page_to_nmdesc(struct page *page)
+{
+	VM_BUG_ON_PAGE(PageTail(page), page);
+	return (struct netmem_desc *)page;
+}
+
+static inline void *nmdesc_address(struct netmem_desc *nmdesc)
+{
+	return page_address(nmdesc_to_page(nmdesc));
+}
+
 /**
  * __netmem_address - unsafely get pointer to the memory backing @netmem
  * @netmem: netmem reference to get the pointer for
-- 
2.17.1