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

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Message-Id: <1181268406.10408.26.camel@localhost.localdomain>
Date:	Thu, 07 Jun 2007 22:06:46 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.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, 2007-06-07 at 18:58 -0700, Andrew Morton wrote:
> On Wed, 06 Jun 2007 23:34:04 -0400

> Interrupts got disabled here because do_page_fault() is an
> interrupt-disabling trap, yes?

Correct.

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

I didn't realize that userspace was allowed to run with interrupts
disabled. If this becomes a problem, we can add the same check that's
above where do_page_fault does enable interrupts, but is skipped because
the faulting address was above PAGE_OFFSET.


ie. (i386)
        if (regs->eflags & (X86_EFLAGS_IF|VM_MASK))
                local_irq_enable();


> 
> However it's surely already the case that most pagefaults will go and
> enable interrupts on this process anyway, so no big loss there.  I'd expect
> the kernel to spit piles of might_sleep() warnings when all this happens, so
> maybe it just doesn't happen for some reason.

Actually it does on the RT kernel. Hence why I found it ;-)

-- Steve


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