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Date:   Tue, 20 Feb 2018 16:16:34 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc:     Dave Hansen <dave.hansen@intel.com>, linux-kernel@vger.kernel.org,
        Christoph Hellwig <hch@infradead.org>, linux-mm@kvack.org,
        Andy Lutomirski <luto@kernel.org>
Subject: Re: [PATCH 3/4] kernel/fork: switch vmapped stack callation to
 __vmalloc_area()
Message-Id: <20180220161634.517598ec63ec4a785c4c81cc@linux-foundation.org>
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On Tue, 23 Jan 2018 16:57:21 +0300 Konstantin Khlebnikov <khlebnikov@yandex-team.ru> wrote:

> # stress-ng --clone 100 -t 10s --metrics-brief
> at 32-core machine shows boost 35000 -> 36000 bogo ops
> 
> Patch 4/4 is a kind of RFC.
> Actually per-cpu cache of preallocated stacks works faster than buddy allocator thus
> performance boots for it happens only at completely insane rate of clones.
> 

I'm not really sure what to make of this patchset.  Is it useful in any
known real-world use cases?

> +	  This option neutralize stack overflow protection but allows to
> +	  achieve best performance for syscalls fork() and clone().

That sounds problematic, but perhaps acceptable if the fallback only
happens rarely.

Can this code be folded into CONFIG_VMAP_STACk in some cleaner fashion?
We now have options for non-vmapped stacks, vmapped stacks and a mix
of both.

And what about this comment in arch/Kconfig:VMAP_STACK:

          This is presently incompatible with KASAN because KASAN expects
          the stack to map directly to the KASAN shadow map using a formula
          that is incorrect if the stack is in vmalloc space.


So VMAP_STACK_AS_FALLBACK will intermittently break KASAN?