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

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Date:	Fri, 13 Feb 2009 13:57:25 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Masami Hiramatsu <mhiramat@...hat.com>
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

* 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 ?

Mathieu

> -- 
> Masami Hiramatsu
> 
> Software Engineer
> Hitachi Computer Products (America) Inc.
> Software Solutions Division
> 
> e-mail: mhiramat@...hat.com
> 

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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