To simplify struct page, the effort to separate its own descriptor from
struct page is required and the work for page pool is on going.

To achieve that, all the code should avoid directly accessing page pool
members of struct page.

Access ->pp_magic through struct netmem_desc instead of directly
accessing it through struct page in page_pool_page_is_pp().  Plus, move
page_pool_page_is_pp() from mm.h to netmem.h to use struct netmem_desc
without header dependency issue.

Signed-off-by: Byungchul Park <byungchul@sk.com>
---
 include/linux/mm.h   | 12 ------------
 include/net/netmem.h | 14 ++++++++++++++
 mm/page_alloc.c      |  1 +
 3 files changed, 15 insertions(+), 12 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8dc012e84033..de10ad386592 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h

@@ -4311,16 +4311,4 @@ 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)
-{
-       return (page->pp_magic & PP_MAGIC_MASK) == PP_SIGNATURE;
-}
-#else
-static inline bool page_pool_page_is_pp(struct page *page)
-{
-       return false;
-}
-#endif
-
 #endif /* _LINUX_MM_H */

diff --git a/include/net/netmem.h b/include/net/netmem.h
index f05a8b008d00..9e4ed3530788 100644
--- a/include/net/netmem.h
+++ b/include/net/netmem.h

@@ -53,6 +53,20 @@ NETMEM_DESC_ASSERT_OFFSET(pp_ref_count, pp_ref_count);
  */
 static_assert(sizeof(struct netmem_desc) <= offsetof(struct page, _refcount));

+#ifdef CONFIG_PAGE_POOL
+static inline bool page_pool_page_is_pp(struct page *page)
+{
+       struct netmem_desc *desc = (__force struct netmem_desc *)page;
+