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

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Date:   Tue, 26 Oct 2021 12:12:16 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Quanyang Wang <quanyang.wang@...driver.com>
Cc:     "Russell King (Oracle)" <linux@...linux.org.uk>,
        Linus Walleij <linus.walleij@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ARM: add BUILD_BUG_ON to check if fixmap range spans
 multiple pmds

On Tue, 26 Oct 2021 at 11:53, Quanyang Wang <quanyang.wang@...driver.com> wrote:
>
> Hi,
> Sorry for the inconvenience.
>
> On 10/26/21 4:59 PM, Russell King (Oracle) wrote:
> > On Sun, Oct 24, 2021 at 11:44:31PM +0200, Linus Walleij wrote:
> >> On Wed, Oct 20, 2021 at 7:50 AM <quanyang.wang@...driver.com> wrote:
> >>
> >>> From: Quanyang Wang <quanyang.wang@...driver.com>
> >>>
> >>> 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 <quanyang.wang@...driver.com>
> >>
> >> Looks reasonable to me.
> >> Reviewed-by: Linus Walleij <linus.walleij@...aro.org>
> >>
> >> 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.