bug in identity map for 4KB pages?

[Ard Biesheuvel]

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);
+
        memblock_enforce_memory_limit(memory_limit);

        /*
"""

(taken from my patch 'arm64: override early_init_dt_add_memory_arch()'
sent to the list a while ago as part of the linear mapping decoupling
series)

-- 
Ard.