linux-kernel - Re: [RFC PATCH 2/2] arm64: Implement vmalloc based thread

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Date:	Sun, 24 May 2015 19:49:01 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Jungseok Lee <jungseoklee85@...il.com>,
	Catalin Marinas <catalin.marinas@....com>, barami97@...il.com,
	Will Deacon <will.deacon@....com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH 2/2] arm64: Implement vmalloc based thread_info allocator

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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