Re: [PATCH 2/2] ARM: LPAE: reduce damage caused by idmap to virtual memory layout

[Konstantin Khlebnikov]

On Mon, Jul 28, 2014 at 10:14 PM, Will Deacon <will.deacon@xxxxxxx> wrote:
> On Tue, Jul 22, 2014 at 04:36:35PM +0100, Konstantin Khlebnikov wrote:
>> idmap layout combines both phisical and virtual addresses.
>> Everything works fine if ram physically lays below PAGE_OFFSET.
>> Otherwise idmap starts punching huge holes in virtual memory layout.
>> It maps ram by 2MiB sections, but when it allocates new pmd page it
>> cuts 1GiB at once.
>>
>> This patch makes a copy of all affected pmds from init_mm.
>> Only few (usually one) 2MiB sections will be lost.
>> This is not eliminates problem but makes it 512 times less likely.
>
> I'm struggling to understand your commit message, but making a problem `512
> times less likely' does sound like a bit of a hack to me. Can't we fix this
> properly instead?

Yep, my comment sucks.

Usually idmap looks like this:

|0x00000000 -- <chunk of physical memory in identical mapping > --- |
TASK_SIZE -- <kernel space vm layoyt> --- 0xFFFFFFFF |

But when that physical memory chunk starts from 0xE8000000 or even
0xF2000000 evenything becomes very complicated.

>
> Will
>
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