mm/migrate: make isolate_movable_page() skip slab pages

In the next commit we want to rearrange struct slab fields to allow a larger
rcu_head. Afterwards, the page->mapping field will overlap with SLUB's "struct
list_head slab_list", where the value of prev pointer can become LIST_POISON2,
which is 0x122 + POISON_POINTER_DELTA.  Unfortunately the bit 1 being set can
confuse PageMovable() to be a false positive and cause a GPF as reported by lkp
[1].

To fix this, make isolate_movable_page() skip pages with the PageSlab flag set.
This is a bit tricky as we need to add memory barriers to SLAB and SLUB's page
allocation and freeing, and their counterparts to isolate_movable_page().

Based on my RFC from [2]. Added a comment update from Matthew's variant in [3]
and, as done there, moved the PageSlab checks to happen before trying to take
the page lock.

[1] https://lore.kernel.org/all/[email protected]/
[2] https://lore.kernel.org/all/[email protected]/
[3] https://lore.kernel.org/all/[email protected]/

Reported-by: kernel test robot <[email protected]>
Signed-off-by: Vlastimil Babka <[email protected]>
Acked-by: Hyeonggon Yoo <[email protected]>

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/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);
 
@@ -2000,8 +2002,10 @@ static void __free_slab(struct kmem_cache *s, struct slab *slab)
 	int pages = 1 << order;
 
 	__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);