[PATCH V4 1/2] slub: fix kmem cgroup bug in kmem_cache_alloc_bulk

[Jesper Dangaard Brouer <brouer@redhat.com>]

The call slab_pre_alloc_hook() interacts with kmemgc and is not
allowed to be called several times inside the bulk alloc for loop,
due to the call to memcg_kmem_get_cache().

This would result in hitting the VM_BUG_ON in __memcg_kmem_get_cache.

As suggested by Vladimir Davydov, change slab_post_alloc_hook()
to be able to handle an array of objects.

A subtle detail is, loop iterator "i" in slab_post_alloc_hook()
must have same type (size_t) as size argument.  This helps the
compiler to easier realize that it can remove the loop, when all
debug statements inside loop evaluates to nothing.
 Note, this is only an issue because the kernel is compiled with
GCC option: -fno-strict-overflow

In slab_alloc_node() the compiler inlines and optimizes the invocation
of slab_post_alloc_hook(s, flags, 1, &object) by removing the loop and
access object directly.

Reported-by: Vladimir Davydov <vdavydov@virtuozzo.com>
Suggested-by: Vladimir Davydov <vdavydov@virtuozzo.com>
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>

---
V4:
 - Adjust slab_alloc_node() to have @object be correct type.
 Did objdump to verify compiler optimized out loop and &addr of @object
 Wrote tool/module to test memory exhaust for error path testing
 [1] https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/mm/slab_bulk_test04_exhaust_mem.c

V3:
 Addressed and used suggetions from Vladimir

 mm/slub.c |   40 ++++++++++++++++++++++------------------
 1 file changed, 22 insertions(+), 18 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 9be12ffae9fc..efcddd223369 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1292,14 +1292,21 @@ static inline struct kmem_cache *slab_pre_alloc_hook(struct kmem_cache *s,
 	return memcg_kmem_get_cache(s, flags);
 }
 
-static inline void slab_post_alloc_hook(struct kmem_cache *s,
-					gfp_t flags, void *object)
+static inline void slab_post_alloc_hook(struct kmem_cache *s, gfp_t flags,
+					size_t size, void **p)
 {
+	size_t i;
+
 	flags &= gfp_allowed_mask;
-	kmemcheck_slab_alloc(s, flags, object, slab_ksize(s));
-	kmemleak_alloc_recursive(object, s->object_size, 1, s->flags, flags);
+	for (i = 0; i < size; i++) {
+		void *object = p[i];
+
+		kmemcheck_slab_alloc(s, flags, object, slab_ksize(s));
+		kmemleak_alloc_recursive(object, s->object_size, 1,
+					 s->flags, flags);
+		kasan_slab_alloc(s, object);
+	}
 	memcg_kmem_put_cache(s);
-	kasan_slab_alloc(s, object);
 }
 
 static inline void slab_free_hook(struct kmem_cache *s, void *x)
@@ -2477,7 +2484,7 @@ static void *__slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
 static __always_inline void *slab_alloc_node(struct kmem_cache *s,
 		gfp_t gfpflags, int node, unsigned long addr)
 {
-	void **object;
+	void *object;
 	struct kmem_cache_cpu *c;
 	struct page *page;
 	unsigned long tid;
@@ -2556,7 +2563,7 @@ redo:
 	if (unlikely(gfpflags & __GFP_ZERO) && object)
 		memset(object, 0, s->object_size);
 
-	slab_post_alloc_hook(s, gfpflags, object);
+	slab_post_alloc_hook(s, gfpflags, 1, &object);
 
 	return object;
 }
@@ -2906,6 +2913,10 @@ bool kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
 	struct kmem_cache_cpu *c;
 	int i;
 
+	/* memcg and kmem_cache debug support */
+	s = slab_pre_alloc_hook(s, flags);
+	if (unlikely(!s))
+		return false;
 	/*
 	 * Drain objects in the per cpu slab, while disabling local
 	 * IRQs, which protects against PREEMPT and interrupts
@@ -2930,17 +2941,8 @@ bool kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
 			c = this_cpu_ptr(s->cpu_slab);
 			continue; /* goto for-loop */
 		}
-
-		/* kmem_cache debug support */
-		s = slab_pre_alloc_hook(s, flags);
-		if (unlikely(!s))
-			goto error;
-
 		c->freelist = get_freepointer(s, object);
 		p[i] = object;
-
-		/* kmem_cache debug support */
-		slab_post_alloc_hook(s, flags, object);
 	}
 	c->tid = next_tid(c->tid);
 	local_irq_enable();
@@ -2953,11 +2955,13 @@ bool kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
 			memset(p[j], 0, s->object_size);
 	}
 
+	/* memcg and kmem_cache debug support */
+	slab_post_alloc_hook(s, flags, size, p);
 	return true;
-
 error:
-	__kmem_cache_free_bulk(s, i, p);
 	local_irq_enable();
+	slab_post_alloc_hook(s, flags, i, p);
+	__kmem_cache_free_bulk(s, i, p);
 	return false;
 }
 EXPORT_SYMBOL(kmem_cache_alloc_bulk);

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