linux-kernel - Re: [PATCH] enable interrupts in user path of page fault.

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Date:	Thu, 7 Jun 2007 19:28:07 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Hellwig <hch@...radead.org>, Andi Kleen <ak@...e.de>
Subject: Re: [PATCH] enable interrupts in user path of page fault.



On Thu, 7 Jun 2007, Andrew Morton wrote:
> 
> Interrupts got disabled here because do_page_fault() is an
> interrupt-disabling trap, yes?

Yes - and it has to be: we want to disable preemption and interrupts that 
can fault on the vmalloc space, until we've at least saved away %cr2. We 
had bugs in that area before.

> The patch looks reasonable to me: a slight reduction in interrupt-off
> latency when really weird things are happening.

I applied it as obviously correct.

> The patch also breaks things, I think: if userspace is running with
> interrupts disabled and tries to access kernel memory it will presently
> whizz through the kernel without ever enabling interrupts.  With this
> change, the kernel will now enable interrupts, which is presumably not what
> the application wanted.

Well, we *do* enable interrupts for real page faults anyway, and this 
whole code just triggers for the case where we'd send a SIGSEGV. If some 
silly app really thought it could do that with interrupts disabled, it was 
wrong before too (we'd hit a reschedule point and enable them there 
anyway).

		Linus
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