[PATCH 06/22] slab: Reimplement page_slab()

["Matthew Wilcox (Oracle)" <willy@infradead.org>]

In order to separate slabs from folios, we need to convert from any page
in a slab to the slab directly without going through a page to folio
conversion first.  page_slab() is a little different from other memdesc
converters we have in that it will return NULL if the page is not part
of a slab.  This will be the normal style for memdesc converters in
the future.

kfence was the only user of page_slab(), so adjust it to the new way
of working.  It will need to be touched again when we separate slab
from page.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 include/linux/page-flags.h | 14 +-------------
 mm/kfence/core.c           | 12 ++++++++----
 mm/slab.h                  | 28 ++++++++++++++++------------
 3 files changed, 25 insertions(+), 29 deletions(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 5ee6ffbdbf83..39ed2f243279 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -1052,19 +1052,7 @@ PAGE_TYPE_OPS(Table, table, pgtable)
  */
 PAGE_TYPE_OPS(Guard, guard, guard)
 
-FOLIO_TYPE_OPS(slab, slab)
-
-/**
- * PageSlab - Determine if the page belongs to the slab allocator
- * @page: The page to test.
- *
- * Context: Any context.
- * Return: True for slab pages, false for any other kind of page.
- */
-static inline bool PageSlab(const struct page *page)
-{
-	return folio_test_slab(page_folio(page));
-}
+PAGE_TYPE_OPS(Slab, slab, slab)
 
 #ifdef CONFIG_HUGETLB_PAGE
 FOLIO_TYPE_OPS(hugetlb, hugetlb)
diff --git a/mm/kfence/core.c b/mm/kfence/core.c
index 0ed3be100963..5fb0f0f8f99a 100644
--- a/mm/kfence/core.c
+++ b/mm/kfence/core.c
@@ -613,12 +613,14 @@ static unsigned long kfence_init_pool(void)
 	 * enters __slab_free() slow-path.
 	 */
 	for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) {
-		struct slab *slab = page_slab(nth_page(pages, i));
+		struct page *page = nth_page(pages, i);
+		struct slab *slab;
 
 		if (!i || (i % 2))
 			continue;
 
-		__folio_set_slab(slab_folio(slab));
+		__SetPageSlab(page);
+		slab = page_slab(page);
 #ifdef CONFIG_MEMCG
 		slab->obj_exts = (unsigned long)&kfence_metadata_init[i / 2 - 1].obj_exts |
 				 MEMCG_DATA_OBJEXTS;
@@ -665,14 +667,16 @@ static unsigned long kfence_init_pool(void)
 
 reset_slab:
 	for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) {
-		struct slab *slab = page_slab(nth_page(pages, i));
+		struct page *page = nth_page(pages, i);
+		struct slab *slab;
 
 		if (!i || (i % 2))
 			continue;
+		slab = page_slab(page);
 #ifdef CONFIG_MEMCG
 		slab->obj_exts = 0;
 #endif
-		__folio_clear_slab(slab_folio(slab));
+		__ClearPageSlab(page);
 	}
 
 	return addr;
diff --git a/mm/slab.h b/mm/slab.h
index bf2f91a6c535..084231394250 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -142,20 +142,24 @@ static_assert(IS_ALIGNED(offsetof(struct slab, freelist), sizeof(freelist_aba_t)
 	struct slab *:		(struct folio *)s))
 
 /**
- * page_slab - Converts from first struct page to slab.
- * @p: The first (either head of compound or single) page of slab.
+ * page_slab - Converts from struct page to its slab.
+ * @page: A page which may or may not belong to a slab.
  *
- * A temporary wrapper to convert struct page to struct slab in situations where
- * we know the page is the compound head, or single order-0 page.
- *
- * Long-term ideally everything would work with struct slab directly or go
- * through folio to struct slab.
- *
- * Return: The slab which contains this page
+ * Return: The slab which contains this page or NULL if the page does
+ * not belong to a slab.  This includes pages returned from large kmalloc.
  */
-#define page_slab(p)		(_Generic((p),				\
-	const struct page *:	(const struct slab *)(p),		\
-	struct page *:		(struct slab *)(p)))
+static inline struct slab *page_slab(const struct page *page)
+{
+	unsigned long head;
+
+	head = READ_ONCE(page->compound_head);
+	if (head & 1)
+		page = (struct page *)(head - 1);
+	if (data_race(page->page_type >> 24) != PGTY_slab)
+		page = NULL;
+
+	return (struct slab *)page;
+}
 
 /**
  * slab_page - The first struct page allocated for a slab
-- 
2.47.2