mm: introduce folio_is_pfmemalloc

Add a folio equivalent for page_is_pfmemalloc. This removes two instances
of page_is_pfmemalloc(folio_page(folio, 0)) so the folio can be used
directly.

Link: https://lkml.kernel.org/r/[email protected]
Suggested-by: Matthew Wilcox <[email protected]>
Signed-off-by: Sidhartha Kumar <[email protected]>
Reviewed-by: Matthew Wilcox (Oracle) <[email protected]>
Reviewed-by: SeongJae Park <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

include/linux/mm.h

@@ -1926,6 +1926,21 @@ static inline bool page_is_pfmemalloc(const struct page *page)
 	return (uintptr_t)page->lru.next & BIT(1);
 }
 
+/*
+ * Return true only if the folio has been allocated with
+ * ALLOC_NO_WATERMARKS and the low watermark was not
+ * met implying that the system is under some pressure.
+ */
+static inline bool folio_is_pfmemalloc(const struct folio *folio)
+{
+	/*
+	 * lru.next has bit 1 set if the page is allocated from the
+	 * pfmemalloc reserves.  Callers may simply overwrite it if
+	 * they do not need to preserve that information.
+	 */
+	return (uintptr_t)folio->lru.next & BIT(1);
+}
+
 /*
  * Only to be called by the page allocator on a freshly allocated
  * page.

mm/slab.c

@@ -1373,7 +1373,7 @@ static struct slab *kmem_getpages(struct kmem_cache *cachep, gfp_t flags,
 	/* Make the flag visible before any changes to folio->mapping */
 	smp_wmb();
 	/* Record if ALLOC_NO_WATERMARKS was set when allocating the slab */
-	if (sk_memalloc_socks() && page_is_pfmemalloc(folio_page(folio, 0)))
+	if (sk_memalloc_socks() && folio_is_pfmemalloc(folio))
 		slab_set_pfmemalloc(slab);
 
 	return slab;

mm/slub.c

@@ -1859,7 +1859,7 @@ static inline struct slab *alloc_slab_page(gfp_t flags, int node,
 	__folio_set_slab(folio);
 	/* Make the flag visible before any changes to folio->mapping */
 	smp_wmb();
-	if (page_is_pfmemalloc(folio_page(folio, 0)))
+	if (folio_is_pfmemalloc(folio))
 		slab_set_pfmemalloc(slab);
 
 	return slab;