[PATCH 1/2] slab: __GFP_ZERO is incompatible with a constructor

[Matthew Wilcox <willy@infradead.org>]

From: Matthew Wilcox <mawilcox@microsoft.com>

__GFP_ZERO requests that the object be initialised to all-zeroes,
while the purpose of a constructor is to initialise an object to a
particular pattern.  We cannot do both.  Add a warning to catch any
users who mistakenly pass a __GFP_ZERO flag when allocating a slab with
a constructor.

Fixes: d07dbea46405 ("Slab allocators: support __GFP_ZERO in all allocators")
Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Cc: stable@vger.kernel.org
---
 mm/slab.c | 6 ++++--
 mm/slob.c | 4 +++-
 mm/slub.c | 6 ++++--
 3 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/mm/slab.c b/mm/slab.c
index 38d3f4fd17d7..8b2cb7db85db 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -3313,8 +3313,10 @@ slab_alloc_node(struct kmem_cache *cachep, gfp_t flags, int nodeid,
 	local_irq_restore(save_flags);
 	ptr = cache_alloc_debugcheck_after(cachep, flags, ptr, caller);
 
-	if (unlikely(flags & __GFP_ZERO) && ptr)
-		memset(ptr, 0, cachep->object_size);
+	if (unlikely(flags & __GFP_ZERO) && ptr) {
+		if (!WARN_ON_ONCE(cachep->ctor))
+			memset(ptr, 0, cachep->object_size);
+	}
 
 	slab_post_alloc_hook(cachep, flags, 1, &ptr);
 	return ptr;
diff --git a/mm/slob.c b/mm/slob.c
index 1a46181b675c..958173fd7c24 100644
--- a/mm/slob.c
+++ b/mm/slob.c
@@ -556,8 +556,10 @@ static void *slob_alloc_node(struct kmem_cache *c, gfp_t flags, int node)
 					    flags, node);
 	}
 
-	if (b && c->ctor)
+	if (b && c->ctor) {
+		WARN_ON_ONCE(flags & __GFP_ZERO);
 		c->ctor(b);
+	}
 
 	kmemleak_alloc_recursive(b, c->size, 1, c->flags, flags);
 	return b;
diff --git a/mm/slub.c b/mm/slub.c
index 9e1100f9298f..0f55f0a0dcaa 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2714,8 +2714,10 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s,
 		stat(s, ALLOC_FASTPATH);
 	}
 
-	if (unlikely(gfpflags & __GFP_ZERO) && object)
-		memset(object, 0, s->object_size);
+	if (unlikely(gfpflags & __GFP_ZERO) && object) {
+		if (!WARN_ON_ONCE(s->ctor))
+			memset(object, 0, s->object_size);
+	}
 
 	slab_post_alloc_hook(s, gfpflags, 1, &object);
 
-- 
2.16.3