linux-kernel - Re: [PATCH 4/6] scs: Move scs_overflow

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Message-ID: <20200518164359.GQ32394@willie-the-truck>
Date:   Mon, 18 May 2020 17:44:00 +0100
From:   Will Deacon <will@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Mark Rutland <mark.rutland@....com>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@...com>,
        Jann Horn <jannh@...gle.com>, Ard Biesheuvel <ardb@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, kernel-team@...roid.com
Subject: Re: [PATCH 4/6] scs: Move scs_overflow_check() out of architecture
 code

On Mon, May 18, 2020 at 08:31:49AM -0700, Kees Cook wrote:
> On Mon, May 18, 2020 at 02:32:31PM +0100, Mark Rutland wrote:
> > On Mon, May 18, 2020 at 02:23:47PM +0100, Will Deacon wrote:
> > > This is something I would like to revisit, but we need more
> > > information from Sami about why Android rejected the larger allocation
> > > size, since I don't think there's an awful lot of point merging this
> > > series if Android doesn't pick it up.
> > 
> > Indeed. I'd certainly prefer the robustness of a VMAP'd SCS if we can do
> > that.
> 
> For smaller devices, the memory overhead was too high. (i.e. 4x more
> memory allocated to kernel stacks -- 4k vs 1k per thread.)

I just don't see an extra 3k per thread as being a real issue (the main
stack is 16k already). Even just the CPU register state is around 1k.

But I'd be very keen to see numbers/performance data that proves me wrong.

Will