linux-kernel - Re: remap allocator for per-CPU memory

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Date:	Wed, 13 May 2009 13:51:22 +0100
From:	"Jan Beulich" <JBeulich@...ell.com>
To:	"Tejun Heo" <TeHeo@...ell.com>
Cc:	"Ingo Molnar" <mingo@...e.hu>, "Andi Kleen" <andi@...stfloor.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: remap allocator for per-CPU memory

>>> Tejun Heo <teheo@...ell.com> 13.05.09 13:29 >>>
>Tejun Heo wrote:
>> Andi Kleen wrote:
>>> Tejun Heo <teheo@...ell.com> writes:
>>>> Okay, just glanced over the pageattr code.  I don't think we need any
>>>> special provisions for this as long as the TLB is fine with having
>>>> overlapping PMD and PTE mappings with different attributes (please
>>>> note that these two mappings aren't occupying the same linear
>>>> addresses - they're aliases).  This is allowed, right?
>>> Nope.
>> 
>> Yeah, I'm going through the manual now and can't find anything which
>> allows such behavior.  I haven't been able to find anything which
>>> describes what happens between large page and 4k page aliases.
>> Aieee...  I'll dig through the manual a bit more and see whether this
>> can be worked around somehow.  :-(
>
>Looks like we're screwed.
>
>I couldn't find anything explicitly prohibiting PMD/PTE aliases w/
>different attributes although there are plenty of warnings and don'ts
>against giving different attributes to the same linear addresses.  At
>any rate, it definitely looks way too dangerous to depend on.
>
>And, set_memory_*() is basically allowed on any memory allocated via
>get_free_page(), so... we're between rock and hard place.  Looks like
>remapping partially using large pages is no go.
>
>Any ideas?

I think the only two alternatives are

(a) don't use large pages in the first place here, or

(b) teach the pageattr code to handle the per-CPU virtual area similarly to
the kernel space for x86-64 (though it's going to be a little more complicated
since there's no pre-determined relation between the virtual and physical
addresses - the necessary lookup might become expensive on systems with
very many [possible] CPUs).

Jan

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