Subject: vmalloc_reserve with no highmem

From: [email-masked] (Laura Abbott)
Date: Tue, 20 Oct 2015 11:52:50 -0700

On 10/19/2015 04:20 PM, Florian Fainelli wrote:
> Hi Russell, Laura,
>
> Setting vmalloc= on the kernel command-line to define the amount of
> vmalloc_reserve is not quite working when you have no highmem, as is the
> case of one my boards which has 512MB or 256M populated on a first bank
> at PA 0x0.
>
> What happens in that case is that, despite setting vmalloc_reserve,
> therefore bumping up vmalloc_min to a higher address than high_memory,
> which is assigned __va(arm_lowmem_limit), we end-up with VMALLOC_START
> at high_memory + VMALLOC_OFFSET, which yields the amount of physical
> memory (start at PA 0x0 in my case) - VMALLOC_OFFSET.
>
> The maths look like this for this particular board (512MB)
>
> high_memory = 0x20000000 + PAGE_OFFSET = 0x20000000 + 0xC0000000 =
> 0xE0000000
> vmalloc_min = 0xFF00000 - (248 * 1024 * 1024) = 0xEF800000
>
> so we end-up with VMALLOC_START = high_memory + VMALLOC_OFFSET =
> 0xE0000000 + 8 * 1024* 1024 = 0xE0800000
>
> in sanity_check_meminfo(), high_memory is unconditionally assigned with
> arm_lowmem_limit's VA.
>
> The following quick and dirty patch seems to do it for me, but I am not
> confident this is remotely the correct approach here:
>
> diff --git a/arch/arm/mm/mmu.c b/arch/arm/mm/mmu.c
> index 14428d2..e196ea4 100644
> --- a/arch/arm/mm/mmu.c
> +++ b/arch/arm/mm/mmu.c
> @@ -1196,7 +1211,14 @@ void __init sanity_check_meminfo(void)
>          }
>   #endif
>          meminfo.nr_banks = j;
> -       high_memory = __va(arm_lowmem_limit - 1) + 1;
> +
> +       if (vmalloc_limit > arm_lowmem_limit)
> +               high_memory = vmalloc_min - VMALLOC_OFFSET;
> +       else
> +               high_memory = __va(arm_lowmem_limit - 1) + 1;
> +
> +       pr_info("%s: high_memory: 0x%p, arm_lowmem_limit: 0x%llx\n",
> +               __func__, high_memory, arm_lowmem_limit);
>
> BTW, even when there is highmem available, setting vmalloc= on the
> command-line is off by VMALLOC_OFFSET, the way early_vmalloc() computes
> things, is that intended?
>
> Thanks!
>

Which baseline are you testing off of? Meminfo was removed
a while ago so I'm not sure where the meminfo.nr_banks context is
coming from.

This is how arm currently defines VMALLOC ranges

#define VMALLOC_OFFSET          (8*1024*1024)
#define VMALLOC_START           (((unsigned long)high_memory + VMALLOC_OFFSET) & ~(VMALLOC_OFFSET-1))
#define VMALLOC_END             0xff000000UL

VMALLOC_START is based off of the value of high_memory. high_memory
is supposed to be the limit of the kernel direct mapped ram region so setting
it to something above that is asking for trouble. It seems like this is
working as intended. You'd have to decouple VMALLOC_START and
high_memory to make this work properly.

Having a vmalloc_start be greater than the end of direct mapped RAM seems a
little unusual. This would be leaving a hole in the virtual space. Is this
what you are intending or is there another use case for adjusting the
vmalloc space as you described?

Thanks,
Laura