linux-kernel - Re: irq-disabled vs vmap vs text

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Date:	Fri, 13 Feb 2009 16:41:57 -0500
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Nick Piggin <npiggin@...e.de>,
	akpm <akpm@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ibm.com>
Subject: Re: irq-disabled vs vmap vs text_poke

Mathieu Desnoyers wrote:
> * Masami Hiramatsu (mhiramat@...hat.com) wrote:
>> Mathieu Desnoyers wrote:
>>>>> text_poke should actually use local_irq_disable/enable rather than
>>>>> local_irq_save/restore because it _must_ be called with interrupts on.
>>>> Could you tell me why it must be called with irq on?
>>>>
>>> Because it uses vmap, but see below..
>>>
>>>>>> Anybody got an idea on how to fix this?
>>>>> There is probably something wrong with the caller, kprobes, which calls
>>>>> text_poke with interrupts off.
>>>> Hmm, kprobe's smoke test caused this problem. Since (un)register_kprobe()
>>>> may sleep for waiting a mutex, it should not be called with interrupts off.
>>>> So, it's not text_poke()'s issue nor vmap().
>>>>
>>>> BTW, what about using map_vm_area() in text_poke() instead of vmap()?
>>>> Since text_poke() just maps text pages to alias pages temporarily,
>>>> I think we don't need to use delayed vunmap().
>>>>
>>> As with this patch from you ? Sorry, when it has been initially
>>> submitted, the discussion turned in a different direction. Please see
>>> comments inline.
>> No, sorry for confusing, vm_unmap_ram() is basically same as vunmap().
>> (The root cause of that bug which I had been reported was not in text_poke())
>>
>> Here I said is (maybe) improving text_poke() by using map_vm_area() which
>> simply maps pages to pre-allocated vm_area. And when unmapping, we just
>> cleanup pte by unmap_kernel_range().
>> For pre-allocating vm_area, we can use get_vm_area().
>>
>> If we can use kmap for this purpose, it will be better solution.
>> However, kmap can not be used for making alias pages.
>>
>> Thanks,
>>
> 
> That sounds clean, and would improve the current way we (mis)use vmap
> for this tiny mapping. I always felt like I was using a sledgehammer
> when only a hammer was necessary. :)
> 
> Do you have a patch that implements this ?

Sure, however, the patch was not well tested (nor designed :-)),
because it has been made as a by-product when I tried to solve
another issue.

Anyway, I'll send it after testing.

Thank you,

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division

e-mail: mhiramat@...hat.com

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