Merge branch 'slab/for-6.2/fit_rcu_head' into slab/for-next

A series by myself to reorder fields in struct slab to allow the
embedded rcu_head to grow (for debugging purposes). Requires changes to
isolate_movable_page() to skip slab pages which can otherwise become
false-positive __PageMovable due to its use of low bits in
page->mapping.

mm/migrate.c

@@ -74,13 +74,22 @@ int isolate_movable_page(struct page *page, isolate_mode_t mode)
 	if (unlikely(!get_page_unless_zero(page)))
 		goto out;
 
+	if (unlikely(PageSlab(page)))
+		goto out_putpage;
+	/* Pairs with smp_wmb() in slab freeing, e.g. SLUB's __free_slab() */
+	smp_rmb();
 	/*
-	 * Check PageMovable before holding a PG_lock because page's owner
-	 * assumes anybody doesn't touch PG_lock of newly allocated page
-	 * so unconditionally grabbing the lock ruins page's owner side.
+	 * Check movable flag before taking the page lock because
+	 * we use non-atomic bitops on newly allocated page flags so
+	 * unconditionally grabbing the lock ruins page's owner side.
 	 */
 	if (unlikely(!__PageMovable(page)))
 		goto out_putpage;
+	/* Pairs with smp_wmb() in slab allocation, e.g. SLUB's alloc_slab_page() */
+	smp_rmb();
+	if (unlikely(PageSlab(page)))
+		goto out_putpage;
+
 	/*
 	 * As movable pages are not isolated from LRU lists, concurrent
 	 * compaction threads can race against page migration functions

mm/slab.c

@@ -1370,6 +1370,8 @@ static struct slab *kmem_getpages(struct kmem_cache *cachep, gfp_t flags,
 
 	account_slab(slab, cachep->gfporder, cachep, flags);
 	__folio_set_slab(folio);
+	/* 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)))
 		slab_set_pfmemalloc(slab);
@@ -1387,9 +1389,11 @@ static void kmem_freepages(struct kmem_cache *cachep, struct slab *slab)
 
 	BUG_ON(!folio_test_slab(folio));
 	__slab_clear_pfmemalloc(slab);
-	__folio_clear_slab(folio);
 	page_mapcount_reset(folio_page(folio, 0));
 	folio->mapping = NULL;
+	/* Make the mapping reset visible before clearing the flag */
+	smp_wmb();
+	__folio_clear_slab(folio);
 
 	if (current->reclaim_state)
 		current->reclaim_state->reclaimed_slab += 1 << order;

mm/slab.h

@@ -11,37 +11,43 @@ struct slab {
 
 #if defined(CONFIG_SLAB)
 
+	struct kmem_cache *slab_cache;
 	union {
-		struct list_head slab_list;
+		struct {
+			struct list_head slab_list;
+			void *freelist;	/* array of free object indexes */
+			void *s_mem;	/* first object */
+		};
 		struct rcu_head rcu_head;
 	};
-	struct kmem_cache *slab_cache;
-	void *freelist;	/* array of free object indexes */
-	void *s_mem;	/* first object */
 	unsigned int active;
 
 #elif defined(CONFIG_SLUB)
 
-	union {
-		struct list_head slab_list;
-		struct rcu_head rcu_head;
-#ifdef CONFIG_SLUB_CPU_PARTIAL
-		struct {
-			struct slab *next;
-			int slabs;	/* Nr of slabs left */
-		};
-#endif
-	};
 	struct kmem_cache *slab_cache;
-	/* Double-word boundary */
-	void *freelist;		/* first free object */
 	union {
-		unsigned long counters;
 		struct {
-			unsigned inuse:16;
-			unsigned objects:15;
-			unsigned frozen:1;
+			union {
+				struct list_head slab_list;
+#ifdef CONFIG_SLUB_CPU_PARTIAL
+				struct {
+					struct slab *next;
+					int slabs;	/* Nr of slabs left */
+				};
+#endif
+			};
+			/* Double-word boundary */
+			void *freelist;		/* first free object */
+			union {
+				unsigned long counters;
+				struct {
+					unsigned inuse:16;
+					unsigned objects:15;
+					unsigned frozen:1;
+				};
+			};
 		};
+		struct rcu_head rcu_head;
 	};
 	unsigned int __unused;
 
@@ -66,9 +72,10 @@ struct slab {
 #define SLAB_MATCH(pg, sl)						\
 	static_assert(offsetof(struct page, pg) == offsetof(struct slab, sl))
 SLAB_MATCH(flags, __page_flags);
-SLAB_MATCH(compound_head, slab_list);	/* Ensure bit 0 is clear */
 #ifndef CONFIG_SLOB
-SLAB_MATCH(rcu_head, rcu_head);
+SLAB_MATCH(compound_head, slab_cache);	/* Ensure bit 0 is clear */
+#else
+SLAB_MATCH(compound_head, slab_list);	/* Ensure bit 0 is clear */
 #endif
 SLAB_MATCH(_refcount, __page_refcount);
 #ifdef CONFIG_MEMCG
@@ -76,6 +83,9 @@ SLAB_MATCH(memcg_data, memcg_data);
 #endif
 #undef SLAB_MATCH
 static_assert(sizeof(struct slab) <= sizeof(struct page));
+#if defined(CONFIG_HAVE_CMPXCHG_DOUBLE) && defined(CONFIG_SLUB)
+static_assert(IS_ALIGNED(offsetof(struct slab, freelist), 2*sizeof(void *)));
+#endif
 
 /**
  * folio_slab - Converts from folio to slab.

mm/slub.c

@@ -1800,6 +1800,8 @@ static inline struct slab *alloc_slab_page(gfp_t flags, int node,
 
 	slab = folio_slab(folio);
 	__folio_set_slab(folio);
+	/* Make the flag visible before any changes to folio->mapping */
+	smp_wmb();
 	if (page_is_pfmemalloc(folio_page(folio, 0)))
 		slab_set_pfmemalloc(slab);
 
@@ -1999,17 +2001,11 @@ static void __free_slab(struct kmem_cache *s, struct slab *slab)
 	int order = folio_order(folio);
 	int pages = 1 << order;
 
-	if (kmem_cache_debug_flags(s, SLAB_CONSISTENCY_CHECKS)) {
-		void *p;
-
-		slab_pad_check(s, slab);
-		for_each_object(p, s, slab_address(slab), slab->objects)
-			check_object(s, slab, p, SLUB_RED_INACTIVE);
-	}
-
 	__slab_clear_pfmemalloc(slab);
-	__folio_clear_slab(folio);
 	folio->mapping = NULL;
+	/* Make the mapping reset visible before clearing the flag */
+	smp_wmb();
+	__folio_clear_slab(folio);
 	if (current->reclaim_state)
 		current->reclaim_state->reclaimed_slab += pages;
 	unaccount_slab(slab, order, s);
@@ -2025,9 +2021,17 @@ static void rcu_free_slab(struct rcu_head *h)
 
 static void free_slab(struct kmem_cache *s, struct slab *slab)
 {
-	if (unlikely(s->flags & SLAB_TYPESAFE_BY_RCU)) {
+	if (kmem_cache_debug_flags(s, SLAB_CONSISTENCY_CHECKS)) {
+		void *p;
+
+		slab_pad_check(s, slab);
+		for_each_object(p, s, slab_address(slab), slab->objects)
+			check_object(s, slab, p, SLUB_RED_INACTIVE);
+	}
+
+	if (unlikely(s->flags & SLAB_TYPESAFE_BY_RCU))
 		call_rcu(&slab->rcu_head, rcu_free_slab);
-	} else
+	else
 		__free_slab(s, slab);
 }