Hi Arnd,
Le 3/10/21 à 6:42 AM, Arnd Bergmann a écrit :
On Thu, Feb 25, 2021 at 12:56 PM Alex Ghiti <alex@xxxxxxxx> wrote:
Le 2/25/21 à 5:34 AM, David Hildenbrand a écrit :
| | | |> +
ffffffc000000000 | -256 GB | ffffffc7ffffffff | 32 GB | kasan
+ ffffffcefee00000 | -196 GB | ffffffcefeffffff | 2 MB | fixmap
+ ffffffceff000000 | -196 GB | ffffffceffffffff | 16 MB | PCI io
+ ffffffcf00000000 | -196 GB | ffffffcfffffffff | 4 GB | vmemmap
+ ffffffd000000000 | -192 GB | ffffffdfffffffff | 64 GB |
vmalloc/ioremap space
+ ffffffe000000000 | -128 GB | ffffffff7fffffff | 126 GB |
direct mapping of all physical memory
^ So you could never ever have more than 126 GB, correct?
I assume that's nothing new.
Before this patch, the limit was 128GB, so in my sense, there is nothing
new. If ever we want to increase that limit, we'll just have to lower
PAGE_OFFSET, there is still some unused virtual addresses after kasan
for example.
Linus Walleij is looking into changing the arm32 code to have the kernel
direct map inside of the vmalloc area, which would be another place
that you could use here. It would be nice to not have too many different
ways of doing this, but I'm not sure how hard it would be to rework your
code, or if there are any downsides of doing this.
This was what my previous version did: https://lkml.org/lkml/2020/6/7/28.
This approach was not welcomed very well and it fixed only the problem
of the implementation of relocatable kernel. The second issue I'm trying
to resolve here is to support both 3 and 4 level page tables using the
same kernel without being relocatable (which would introduce performance
penalty). I can't do it when the kernel mapping is in the vmalloc region
since vmalloc region relies on PAGE_OFFSET which is different on both 3
and 4 level page table and that would then require the kernel to be
relocatable.
Alex
Arnd
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