linux-kernel - Re: arch/x86/mm/ioremap unification grew by 10x

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Date:	Mon, 18 Feb 2008 04:17:17 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"Huang, Ying" <ying.huang@...el.com>
Cc:	Ian Campbell <ijc@...lion.org.uk>, Matt Mackall <mpm@...enic.com>,
	Sam Ravnborg <sam@...nborg.org>,
	Linux Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: arch/x86/mm/ioremap unification grew by 10x


* Huang, Ying <ying.huang@...el.com> wrote:

> > > And here's most of the cause:
> > > 
> > > 000002b8 00000124 T early_ioremap
> > > 00001000 00001000 t bm_pte
> > > 00002000 00000004 T early_ioremap_debug
> > > 
> > > static __initdata pte_t bm_pte[PAGE_SIZE/sizeof(pte_t)]
> > > 				__attribute__((aligned(PAGE_SIZE)));
> > > 
> > > Double ouch. First, this isn't in BSS. Second, even though it's
> > > initdata, the alignment slop won't get recovered.
> > 
> > 551889a6e2a24a9c06fd453ea03b57b7746ffdc0 (the reason you CC'd me) only
> > changed this from unsigned long[1024] to pte_t[PAGE_SIZE/sizeof(pte_t)]
> > 
> > -static __initdata unsigned long bm_pte[1024]
> > +static __initdata pte_t bm_pte[PAGE_SIZE/sizeof(pte_t)]
> >                                 __attribute__((aligned(PAGE_SIZE)));
> > 
> > I don't think that would have changed anything in this respect. It seems
> > to have been that way since 0947b2f31ca1ea1211d3cde2dbd8fcec579ef395
> > when it was added (Huang Ying CC'd).
> > 
> > > Don't we have a special section for page-aligned crap so it doesn't
> > > waste most of two pages?
> > 
> > We have .bss.page_aligned and it seems appropriate to use it.
> 
> But .bss.page_aligned is not an init section, this will waste some 
> memory after boot. Do you think that it is more appropriate to add 
> another section like .init.bss and .init.bss.page_aligned.

agreed, and i'd suggest we do so before this information gets forgotten 
and someone has to re-discover the fix the hard way.

	Ingo
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