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Subject[PATCH 2/3] kernel: irq: use a kmem_cache for allocating struct irq_desc
After enabling alignment checks in UBSan I've noticed a lot of
reports like this:

UBSan: Undefined behaviour in ../kernel/irq/chip.c:195:14
member access within misaligned address ffff88003e80d6f8
for type 'struct irq_desc' which requires 16 byte alignment

struct irq_desc declared with ____cacheline_internodealigned_in_smp
attribute. However in some cases it allocated dynamically via kmalloc().
In general case kmalloc() guaranties only sizeof(void *) alignment.
We should use a separate slab cache to make struct irq_desc
properly aligned on SMP configuration.

This also could slightly reduce memory usage on some configurations.
E.g. in my setup sizeof(struct irq_desc) == 320. Which means that
kmalloc-512 will be used for allocating irg_desc via kmalloc().
In that case using separate slab cache will save us 192 bytes per
each irq_desc.

Note: UBSan reports says that 'struct irq_desc' requires 16 byte alignment.
It's wrong, in my setup it should be 64 bytes. This looks like a gcc bug,
but it doesn't change the fact that irq_desc is misaligned.

Signed-off-by: Andrey Ryabinin <>
kernel/irq/irqdesc.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/irq/irqdesc.c b/kernel/irq/irqdesc.c
index a1782f8..f22cb87 100644
--- a/kernel/irq/irqdesc.c
+++ b/kernel/irq/irqdesc.c
@@ -23,6 +23,8 @@
static struct lock_class_key irq_desc_lock_class;

+static struct kmem_cache *irq_desc_cachep;
#if defined(CONFIG_SMP)
static void __init init_irq_default_affinity(void)
@@ -137,9 +139,10 @@ static struct irq_desc *alloc_desc(int irq, int node, struct module *owner)
struct irq_desc *desc;
gfp_t gfp = GFP_KERNEL;

- desc = kzalloc_node(sizeof(*desc), gfp, node);
+ desc = kmem_cache_zalloc_node(irq_desc_cachep, gfp, node);
if (!desc)
return NULL;
/* allocate based on nr_cpu_ids */
desc->kstat_irqs = alloc_percpu(unsigned int);
if (!desc->kstat_irqs)
@@ -158,7 +161,7 @@ static struct irq_desc *alloc_desc(int irq, int node, struct module *owner)
- kfree(desc);
+ kmem_cache_free(irq_desc_cachep, desc);
return NULL;

@@ -174,7 +177,7 @@ static void free_desc(unsigned int irq)

- kfree(desc);
+ kmem_cache_free(irq_desc_cachep, desc);

static int alloc_descs(unsigned int start, unsigned int cnt, int node,
@@ -218,6 +221,8 @@ int __init early_irq_init(void)


+ irq_desc_cachep = KMEM_CACHE(irq_desc, SLAB_PANIC);
/* Let arch update nr_irqs and return the nr of preallocated irqs */
initcnt = arch_probe_nr_irqs();
printk(KERN_INFO "NR_IRQS:%d nr_irqs:%d %d\n", NR_IRQS, nr_irqs, initcnt);

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