[PATCH 1/3] mm/slub: hw_tags: skip page-allocator unpoisoning on slab allocation

[Dev Jain <dev.jain@arm.com>]

When a new slab page is allocated, the buddy will unpoison the page.
Then slab immediately poisons the page via kasan_poison_slab(). This
is wasted work.

Similar to what is done in vmalloc currently, use GFP_SKIP_KASAN
(hw tags flag only) to skip unpoisoning of the slab page.

Signed-off-by: Dev Jain <dev.jain@arm.com>
---
 mm/page_alloc.c |  2 +-
 mm/slub.c       | 11 +++++++++--
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 227d58dc3de6..c3a69913aaa9 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -7723,7 +7723,7 @@ struct page *alloc_frozen_pages_nolock_noprof(gfp_t gfp_flags, int nid, unsigned
 	struct alloc_context ac = { };
 	struct page *page;
 
-	VM_WARN_ON_ONCE(gfp_flags & ~__GFP_ACCOUNT);
+	VM_WARN_ON_ONCE(gfp_flags & ~(__GFP_ACCOUNT | __GFP_SKIP_KASAN));
 	/*
 	 * In PREEMPT_RT spin_trylock() will call raw_spin_lock() which is
 	 * unsafe in NMI. If spin_trylock() is called from hard IRQ the current
diff --git a/mm/slub.c b/mm/slub.c
index 0baa906f39ab..da3520769d1f 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3269,9 +3269,16 @@ static inline struct slab *alloc_slab_page(gfp_t flags, int node,
 	struct slab *slab;
 	unsigned int order = oo_order(oo);
 
+	/*
+	 * New slab pages are immediately poisoned by kasan_poison_slab()
+	 * before any object is handed out, so page allocator unpoisoning
+	 * is wasted work for HW_TAGS KASAN.
+	 */
+	flags |= __GFP_SKIP_KASAN;
+
 	if (unlikely(!allow_spin))
-		page = alloc_frozen_pages_nolock(0/* __GFP_COMP is implied */,
-								  node, order);
+		page = alloc_frozen_pages_nolock(__GFP_SKIP_KASAN,
+						 node, order);
 	else if (node == NUMA_NO_NODE)
 		page = alloc_frozen_pages(flags, order);
 	else
-- 
2.43.0