linux-kernel - Re: One (possible) x86 get_user

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Date:	Thu, 27 Jan 2011 17:25:02 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Jan Beulich <JBeulich@...ell.com>
Cc:	Xiaowei Yang <xiaowei.yang@...wei.com>,
	Nick Piggin <npiggin@...nel.dk>, fanhenglong@...wei.com,
	Kaushik Barde <kbarde@...wei.com>,
	Kenneth Lee <liguozhu@...wei.com>,
	linqaingmin <linqiangmin@...wei.com>, wangzhenguo@...wei.com,
	Wu Fengguang <fengguang.wu@...el.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	linux-kernel@...r.kernel.org
Subject: Re: One (possible) x86 get_user_pages bug

On Thu, 2011-01-27 at 16:07 +0000, Jan Beulich wrote:
> 
> Nick, based on your doing of the initial implementation, would
> you be able to estimate whether disabling get_user_pages_fast()
> altogether for Xen would be performing measurably worse than
> adding the locks (but continuing to avoid acquiring mm->mmap_sem)
> as suggested by Xiaowei? That's of course only if the latter is correct
> at all, of which I haven't fully convinced myself yet. 

Adding the lock will result in deadlocks, __get_user_pages_fast() is
used from NMI context.

Then again, I've got no clue if Xen even has NMIs..
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