[RFC 1/2] mm: Use page->mapping to indicate pfmemalloc

[Matthew Wilcox]

From: Matthew Wilcox <mawil...@microsoft.com>

I want to use page->index for a different purpose, so move the pfmemalloc
indicator from page->index to page->mapping.

Signed-off-by: Matthew Wilcox <mawil...@microsoft.com>
---
 include/linux/mm.h | 16 +++++-----------
 mm/page_alloc.c    |  8 +++-----
 2 files changed, 8 insertions(+), 16 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 4ef7fb1726ab..06ea71358bda 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1122,6 +1122,9 @@ static inline pgoff_t page_index(struct page *page)
 bool page_mapped(struct page *page);
 struct address_space *page_mapping(struct page *page);
 
+/* page->mapping cannot point into the zero page. */
+#define MAPPING_PFMEMALLOC     ((struct address_space *)4)
+
 /*
  * Return true only if the page has been allocated with
  * ALLOC_NO_WATERMARKS and the low watermark was not
@@ -1129,11 +1132,7 @@ struct address_space *page_mapping(struct page *page);
  */
 static inline bool page_is_pfmemalloc(struct page *page)
 {
-       /*
-        * Page index cannot be this large so this must be
-        * a pfmemalloc page.
-        */
-       return page->index == -1UL;
+       return page->mapping == MAPPING_PFMEMALLOC;
 }
 
 /*
@@ -1142,12 +1141,7 @@ static inline bool page_is_pfmemalloc(struct page *page)
  */
 static inline void set_page_pfmemalloc(struct page *page)
 {
-       page->index = -1UL;
-}
-
-static inline void clear_page_pfmemalloc(struct page *page)
-{
-       page->index = 0;
+       page->mapping = MAPPING_PFMEMALLOC;
 }
 
 /*
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 796ce1b3e0a1..7a9c14214ed2 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1047,7 +1047,7 @@ static __always_inline bool free_pages_prepare(struct 
page *page,
                        (page + i)->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
                }
        }
-       if (PageMappingFlags(page))
+       if (PageMappingFlags(page) || page_is_pfmemalloc(page))
                page->mapping = NULL;
        if (memcg_kmem_enabled() && PageKmemcg(page))
                memcg_kmem_uncharge(page, order);
@@ -1802,8 +1802,8 @@ inline void post_alloc_hook(struct page *page, unsigned 
int order,
        set_page_owner(page, order, gfp_flags);
 }
 
-static void prep_new_page(struct page *page, unsigned int order, gfp_t 
gfp_flags,
-                                                       unsigned int 
alloc_flags)
+static void prep_new_page(struct page *page, unsigned int order,
+                               gfp_t gfp_flags, unsigned int alloc_flags)
 {
        int i;
 
@@ -1824,8 +1824,6 @@ static void prep_new_page(struct page *page, unsigned int 
order, gfp_t gfp_flags
         */
        if (alloc_flags & ALLOC_NO_WATERMARKS)
                set_page_pfmemalloc(page);
-       else
-               clear_page_pfmemalloc(page);
 }
 
 /*
-- 
2.16.2