linux-kernel - Re: [PATCH 05/14] arm64: clean up THREAD

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Message-ID: <20170814131023.GA23428@leverpostej>
Date:   Mon, 14 Aug 2017 14:10:24 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, keescook@...omium.org,
        ard.biesheuvel@...aro.org, matt@...eblueprint.co.uk,
        kernel-hardening@...ts.openwall.com, will.deacon@....com,
        linux-kernel@...r.kernel.org, luto@...capital.net,
        james.morse@....com, labbott@...hat.com
Subject: Re: [PATCH 05/14] arm64: clean up THREAD_* definitions

On Mon, Aug 14, 2017 at 12:59:59PM +0100, Catalin Marinas wrote:
> On Mon, Aug 07, 2017 at 07:35:56PM +0100, Mark Rutland wrote:
> > Currently we define THREAD_SIZE and THREAD_SIZE order separately, with
> 
> s/THREAD_SIZE order/THREAD_SIZE_ORDER/

Whoops; I'll fix that up now.

> > +#define THREAD_SHIFT		14
> > +
> > +#if THREAD_SHIFT >= PAGE_SHIFT
> > +#define THREAD_SIZE_ORDER	(THREAD_SHIFT - PAGE_SHIFT)
> > +#endif
> > +
> > +#define THREAD_SIZE		(UL(1) << THREAD_SHIFT)
> 
> I haven't tried the series to this patch but it seems to me that
> THREAD_SIZE_ORDER is undefined for a PAGE_SHIFT of 16. 

That is already the case without these patches, as we have:

#ifdef CONFIG_ARM64_4K_PAGES
#define THREAD_SIZE_ORDER       2
#elif defined(CONFIG_ARM64_16K_PAGES)
#define THREAD_SIZE_ORDER       0
#endif

... this is also deliberate, as we'd need an order of -2 with 16K stacks and
64K pages, and this doesn't make sense.

THREAD_SIZE_ORDER only matters if THREAD_SIZE >= PAGE_SIZE (or if we're using a
VMAP'd stack). For 64K pages && VMAP_STACK, we'll use a 64K stack, avoiding the
problem.

Thanks,
Mark.