bug in identity map for 4KB pages?

[Ard Biesheuvel]

On 29 July 2015 at 14:53, Mark Rutland <mark.rutland at arm.com> wrote:
> On Wed, Jul 29, 2015 at 12:49:53PM +0100, Ard Biesheuvel wrote:
>> On 29 July 2015 at 13:42, Mark Rutland <mark.rutland at arm.com> wrote:
>> > On Wed, Jul 29, 2015 at 08:47:10AM +0100, Ard Biesheuvel wrote:
>> >> On 29 July 2015 at 04:37, Stuart Yoder <stuart.yoder at freescale.com> wrote:
>> >> > Our system has RAM at a high address, and previously required using 48-bit VA
>> >> > in order to have an idmap that covered all of RAM.
>> >> >
>> >> > In testing on 4.2-rc4, which now contains support for the increased VA range
>> >> > of the idmap without using 48-bit VA, I'm finding that things work for
>> >> > 64KB pages, but do not for 4KB pages.
>> >> >
>> >> > Is there any known limitation here with 4KB pages?  Any ideas?
>> >> >
>> >>
>> >> You probably have memory at 0x8000_0000 and at 0x80_8000_0000, right?
>> >> So the physical arrangement still requires more than the 39 bits of
>> >> virtual address space you get with 3 levels, even if the ID map can
>> >> cope now. That is why you get the 0x40_0000_0000 virtual address:
>> >> __phys_to_virt() just wraps to a positive number.
>> >
>> > Ah, I see. So it's the linear mapping rather than the idmap which is the
>> > problem.
>> >
>> > We cut memory which would fall below the start of the linear map in
>> > early_init_dt_add_memory_arch, by cutting memory below phys_offset.
>> >
>> > It looks like we already have the logic for cutting memory beyond the
>> > end of the linear map, so we should just need to override the limit.
>> >
>> > Stuart, does the below patch prevent the panic you see?
>> >
>>
>> Wouldn't something like this make more sense?
>>
>> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
>> index 597831bdddf3..64480b65ef17 100644
>> --- a/arch/arm64/mm/init.c
>> +++ b/arch/arm64/mm/init.c
>> @@ -158,6 +158,15 @@ early_param("mem", early_mem);
>>
>>  void __init arm64_memblock_init(void)
>>  {
>> +       /*
>> +        * Remove the memory that we will not be able to cover
>> +        * with the linear mapping.
>> +        */
>> +       const s64 linear_region_size = -(s64)PAGE_OFFSET;
>> +
>> +       memblock_remove(0, memstart_addr);
>> +       memblock_remove(memstart_addr + linear_region_size, ULLONG_MAX);
>
> The maths is more correct, certainly.
>
> I'd prefer to handle the truncation in early_init_dt_add_memory_arch
> because it already has the requisite logic, and keeping the truncation
> in one place should keep things less confusing.
>
> To that end I'll respin my patch using PAGE_OFFSET to determine the
> physical memory limit.
>
> Unless there's something I've missed?
>

I don't think so. But perhaps it is just as easy just to override
early_init_dt_add_memory_arch() now since I will need to do that
anyway if the linear mapping changes ever land.

http://article.gmane.org/gmane.linux.kernel.efi/5744