linux-kernel - Re: [patch V3 05/37] asm-generic: Provide kmap

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Date:   Tue, 3 Nov 2020 13:25:27 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
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Subject: Re: [patch V3 05/37] asm-generic: Provide kmap_size.h

On Tue, Nov 3, 2020 at 10:27 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> kmap_types.h is a misnomer because the old atomic MAP based array does not
> exist anymore and the whole indirection of architectures including
> kmap_types.h is inconinstent and does not allow to provide guard page
> debugging for this misfeature.
>
> Add a common header file which defines the mapping stack size for all
> architectures. Will be used when converting architectures over to a
> generic kmap_local/atomic implementation.
>
> The array size is chosen with the following constraints in mind:
>
>     - The deepest nest level in one context is 3 according to code
>       inspection.
>
>     - The worst case nesting for the upcoming reemptible version would be:
>
>       2 maps in task context and a fault inside
>       2 maps in the fault handler
>       3 maps in softirq
>       2 maps in interrupt
>
> So a total of 16 is sufficient and probably overestimated.
>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

Acked-by: Arnd Bergmann <arnd@...db.de>