Thread (13 messages) 13 messages, 5 authors, 2021-10-31

Re: [PATCH] ARM: add BUILD_BUG_ON to check if fixmap range spans multiple pmds

From: Ard Biesheuvel <ardb@kernel.org>
Date: 2021-10-26 10:12:32
Also in: lkml

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

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