[1/6] powerpc/mm/radix: Fix off-by-one in split mapping logic

[Michael Ellerman]

On Fri, 2018-10-19 at 04:13:29 UTC, Michael Ellerman wrote:
> When we have CONFIG_STRICT_KERNEL_RWX enabled, we try to split the
> kernel linear (1:1) mapping so that the kernel text is in a separate
> page to kernel data, so we can mark the former read-only.
> 
> We could achieve that just by always using 64K pages for the linear
> mapping, but we try to be smarter. Instead we use huge pages when
> possible, and only switch to smaller pages when necessary.
> 
> However we have an off-by-one bug in that logic, which causes us to
> calculate the wrong boundary between text and data.
> 
> For example with the end of the kernel text at 16M we see:
> 
>   radix-mmu: Mapped 0x0000000000000000-0x0000000001200000 with 64.0 KiB pages
>   radix-mmu: Mapped 0x0000000001200000-0x0000000040000000 with 2.00 MiB pages
>   radix-mmu: Mapped 0x0000000040000000-0x0000000100000000 with 1.00 GiB pages
> 
> ie. we mapped from 0 to 18M with 64K pages, even though the boundary
> between text and data is at 16M.
> 
> With the fix we see we're correctly hitting the 16M boundary:
> 
>   radix-mmu: Mapped 0x0000000000000000-0x0000000001000000 with 64.0 KiB pages
>   radix-mmu: Mapped 0x0000000001000000-0x0000000040000000 with 2.00 MiB pages
>   radix-mmu: Mapped 0x0000000040000000-0x0000000100000000 with 1.00 GiB pages
> 
> Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>

Series applied to powerpc next.

https://git.kernel.org/powerpc/c/5c6499b7041b43807dfaeda28aa87f

cheers