Hi Ard,

On 10/26/21 6:12 PM, Ard Biesheuvel wrote:

On Tue, 26 Oct 2021 at 11:53, Quanyang Wang [off-list ref] wrote:

quoted

Hi,
Sorry for the inconvenience.

On 10/26/21 4:59 PM, Russell King (Oracle) wrote:

quoted

On Sun, Oct 24, 2021 at 11:44:31PM +0200, Linus Walleij wrote:

quoted

On Wed, Oct 20, 2021 at 7:50 AM [off-list ref] wrote:

quoted

From: Quanyang Wang <redacted>

Not only the early fixmap range, but also the fixmap range should be
checked if it spans multiple pmds. When enabling CONFIG_DEBUG_HIGHMEM,
some systems which contain up to 16 CPUs will crash.

Signed-off-by: Quanyang Wang <redacted>

Looks reasonable to me.
Reviewed-by: Linus Walleij <redacted>

Please submit this patch into Russell's patch tracker.

... and has totally broken what looks like _all_ ARM kernel builds.

This patch is intended to trigger build error when it check the value of
__end_of_fixmap_region is equal or larger than 256.

Why? The fixmap region is larger than one PMD, so why do we need to cap it?

In __kmap_local_pfn_prot, arch_kmap_local_set_pte(&init_mm, vaddr, 
kmap_pte - idx, pteval) is used to set pteval.
But the ptep is calculated by "kmap_pte - idx", which means all ptes 
must be placed next to each other and no gaps. But for ARM, the ptes for 
the range "0xffe00000~0xfff00000" is not next to the ptes for the range
"0xffc80000~0xffdfffff".

When the idx is larger than 256, virtual address is in 0xffdxxxxx, 
access this address will crash since its pteval isn't set correctly.

quoted

In fact, it breaks the ARM kernel builds which NR_CPUS is equal or more
than 16. If CONFIG_DEBUG_HIGHMEM is enabled, all ARM builds which
NR_CPUS is more than 8 will fail.

You really need to be more specific about the failure mode here.

OK, I will be more careful about this.

Thanks,
Quanyang

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