[PATCH 2/2] mm: Indicate pfmemalloc pages in compound_head

[Matthew Wilcox (Oracle)]

The net page_pool wants to use a magic value to identify page pool pages.
The best place to put it is in the first word where it can be clearly a
non-pointer value.  That means shifting dma_addr up to alias with ->index,
which means we need to find another way to indicate page_is_pfmemalloc().
Since page_pool doesn't want to set its magic value on pages which are
pfmemalloc, we can use bit 1 of compound_head to indicate that the page
came from the memory reserves.

Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
---
 include/linux/mm.h       | 12 +++++++-----
 include/linux/mm_types.h |  7 +++----
 2 files changed, 10 insertions(+), 9 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8ba434287387..44eab3f6d5ae 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1629,10 +1629,12 @@ struct address_space *page_mapping_file(struct page *page);
 static inline bool page_is_pfmemalloc(const struct page *page)
 {
 	/*
-	 * Page index cannot be this large so this must be
-	 * a pfmemalloc page.
+	 * This is not a tail page; compound_head of a head page is unused
+	 * at return from the page allocator, and will be overwritten
+	 * by callers who do not care whether the page came from the
+	 * reserves.
 	 */
-	return page->index == -1UL;
+	return page->compound_head & 2;
 }
 
 /*
@@ -1641,12 +1643,12 @@ static inline bool page_is_pfmemalloc(const struct page *page)
  */
 static inline void set_page_pfmemalloc(struct page *page)
 {
-	page->index = -1UL;
+	page->compound_head = 2;
 }
 
 static inline void clear_page_pfmemalloc(struct page *page)
 {
-	page->index = 0;
+	page->compound_head = 0;
 }
 
 /*
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 5aacc1c10a45..39f7163dcace 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -96,10 +96,9 @@ struct page {
 			unsigned long private;
 		};
 		struct {	/* page_pool used by netstack */
-			/**
-			 * @dma_addr: might require a 64-bit value on
-			 * 32-bit architectures.
-			 */
+			unsigned long pp_magic;
+			unsigned long xmi;
+			unsigned long _pp_mapping_pad;
 			unsigned long dma_addr[2];
 		};
 		struct {	/* slab, slob and slub */
-- 
2.30.2