From: Arnd Bergmann <arnd@arndb.de>
To: linux-arm-kernel@lists.infradead.org
Cc: Jungseok Lee <jungseoklee85@gmail.com>,
        Catalin Marinas <catalin.marinas@arm.com>, barami97@gmail.com,
        Will Deacon <will.deacon@arm.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [RFC PATCH 2/2] arm64: Implement vmalloc based thread_info allocator
Date: Sun, 24 May 2015 19:49:01 +0200
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On Monday 25 May 2015 01:02:20 Jungseok Lee wrote:
> Fork-routine sometimes fails to get a physically contiguous region for
> thread_info on 4KB page system although free memory is enough. That is,
> a physically contiguous region, which is currently 16KB, is not available
> since system memory is fragmented.
> 
> This patch tries to solve the problem as allocating thread_info memory
> from vmalloc space, not 1:1 mapping one. The downside is one additional
> page allocation in case of vmalloc. However, vmalloc space is large enough,
> around 240GB, under a combination of 39-bit VA and 4KB page. Thus, it is
> not a big tradeoff for fork-routine service.

vmalloc has a rather large runtime cost. I'd argue that failing to allocate
thread_info structures means something has gone very wrong.

Can you describe the scenario that leads to fragmentation this bad?

Could the stack size be reduced to 8KB perhaps?

	Arnd
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