Re: block: DMA alignment of IO buffer allocated from slab

[Ming Lei]

On Fri, Sep 21, 2018 at 03:04:18PM +0200, Vitaly Kuznetsov wrote:
> 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).

Even some of buffer address is _not_ L1 cache size aligned, this way is
totally broken wrt. DMA to/from this buffer.

So this issue has to be fixed in slab debug side.

Thanks,
Ming