Christoph Hellwig <hch@xxxxxx> writes:
> On Wed, Sep 19, 2018 at 05:15:43PM +0800, Ming Lei wrote:
>> 1) does kmalloc-N slab guarantee to return N-byte aligned buffer? If
>> yes, is it a stable rule?
>
> This is the assumption in a lot of the kernel, so I think if somethings
> breaks this we are in a lot of pain.
It seems that SLUB debug breaks this assumption. Kernel built with
CONFIG_SLUB_DEBUG=y
CONFIG_SLUB=y
CONFIG_SLUB_DEBUG_ON=y
And the following patch:
diff --git a/arch/x86/kernel/acpi/boot.c b/arch/x86/kernel/acpi/boot.c
index 3b20607d581b..56713b201921 100644
--- a/arch/x86/kernel/acpi/boot.c
+++ b/arch/x86/kernel/acpi/boot.c
@@ -1771,3 +1771,28 @@ void __init arch_reserve_mem_area(acpi_physical_address addr, size_t size)
e820__range_add(addr, size, E820_TYPE_ACPI);
e820__update_table_print();
}
+
+#define KMALLOCS 16
+
+static __init int kmalloc_check_512(void)
+{
+ void *buf[KMALLOCS];
+ int i;
+
+ pr_info("kmalloc_check_512: start\n");
+
+ for (i = 0; i < KMALLOCS; i++) {
+ buf[i] = kmalloc(512, GFP_KERNEL);
+ }
+
+ for (i = 0; i < KMALLOCS; i++) {
+ pr_info("%lx %x\n", (unsigned long)buf[i], ((unsigned long)buf[i]) % 512);
+ kfree(buf[i]);
+ }
+
+ pr_info("kmalloc_check_512: done\n");
+
+ return 0;
+}
+
+late_initcall(kmalloc_check_512);
gives me the following output:
[ 8.417468] kmalloc_check_512: start
[ 8.429572] ffff9a3258bb09f8 1f8
[ 8.435513] ffff9a3258bb70a8 a8
[ 8.441352] ffff9a3258bb0d48 148
[ 8.447139] ffff9a3258bb6d58 158
[ 8.452864] ffff9a3258bb1098 98
[ 8.458536] ffff9a3258bb6a08 8
[ 8.464103] ffff9a3258bb13e8 1e8
[ 8.469534] ffff9a3258bb66b8 b8
[ 8.474907] ffff9a3258bb1738 138
[ 8.480214] ffff9a3258bb6368 168
[ 8.480217] ffff9a3258bb1a88 88
[ 8.496178] ffff9a3258bb6018 18
[ 8.501218] ffff9a3258bb1dd8 1d8
[ 8.506138] ffff9a3258bb5cc8 c8
[ 8.511010] ffff9a3258bb2128 128
[ 8.515795] ffff9a3258bb5978 178
[ 8.520517] kmalloc_check_512: done
(without SLUB_DEBUG_ON all addresses are 512b aligned).
--
Vitaly
