linux-kernel - Re: + kmap-types-clean-up-and-optimization.patch added to -mm tree

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Date:	Thu, 23 Dec 2010 10:47:42 +0000
From:	"Jan Beulich" <JBeulich@...ell.com>
To:	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>
Cc:	<mingo@...e.hu>, <tglx@...utronix.de>, <akpm@...ux-foundation.org>,
	"LKML" <linux-kernel@...r.kernel.org>,
	<mm-commits@...r.kernel.org>, <hpa@...or.com>
Subject: Re: + kmap-types-clean-up-and-optimization.patch added to -mm
	 tree

>>> On 23.12.10 at 10:58, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> On Wed, 2010-12-22 at 14:48 -0800, akpm@...ux-foundation.org wrote:
>> The patch titled
>>      kmap-types: clean up and optimization
>> has been added to the -mm tree.  Its filename is
>>      kmap-types-clean-up-and-optimization.patch
>> 
>> Before you just go and hit "reply", please:
>>    a) Consider who else should be cc'ed
>>    b) Prefer to cc a suitable mailing list as well
>>    c) Ideally: find the original patch on the mailing list and do a
>>       reply-to-all to that, adding suitable additional cc's
>> 
> 
>> ------------------------------------------------------
>> Subject: kmap-types: clean up and optimization
>> From: "Jan Beulich" <JBeulich@...ell.com>
>> 
>> Several of the types aren't being used at all anymore - those can be
>> deleted altogether.  
> 
> NO! That's wrong, in fact non of them are being used, but removing them
> will decrease the number of kmap_atomic slots, but those are still being
> used.

Would you mind pointing out examples of such uses (i.e. without
the proper enumerator)? How would those avoid collisions with
actually used slots?

>> Others are used only by single components that can be
>> assumed to be enabled everywhere, so those are made dependent upon
>> CONFIG_* settings.  Since this somewhat conflicts with the sequential gap
>> markers used under __WITH_KM_FENCE, and since this can be simplified
>> anyway, fold the enumerator definitions with the (modified accordingly)
>> KMAP_D() macro always.
>> 
>> The whole point of the reduction is that, at least on ix86, the number of
>> kmap types can (depending on configuration) affect the amount of low
>> memory, and thus unused types should be avoided if possible.
> 
> Feh, its only a few pages and since there is no way to actually tell if
> you've got enough kmap atomic pages other than experiencing runtime
> errors, removing them must be done with utmost prudence.

Whether 2Mb of lowmem is "only a few pages" certainly depends
on the perspective you take.

And even then - shouldn't the bad (non-enumerated) uses of
atomic kmap-s be fixed rather than keeping unused entries in
the enumeration just because there is broken code somewhere?

Jan

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