linux-kernel - Re: [PATCH 3/3] reuse __free_pages_exact() in __alloc_pages

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Date:	Tue, 12 Apr 2011 08:24:24 -0700
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Michal Nazarewicz <mina86@...a86.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Timur Tabi <timur@...escale.com>,
	Andi Kleen <andi@...stfloor.org>, Mel Gorman <mel@....ul.ie>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH 3/3] reuse __free_pages_exact() in __alloc_pages_exact()

On Tue, 2011-04-12 at 12:29 +0200, Michal Nazarewicz wrote:
> On Tue, 12 Apr 2011 00:03:48 +0200, Dave Hansen <dave@...ux.vnet.ibm.com>  
> wrote:
> > diff -puN mm/page_alloc.c~reuse-free-exact mm/page_alloc.c
> > --- linux-2.6.git/mm/page_alloc.c~reuse-free-exact	2011-04-11  
> > 15:01:17.701822598 -0700
> > +++ linux-2.6.git-dave/mm/page_alloc.c	2011-04-11 15:01:17.713822594  
> > -0700
> > @@ -2338,14 +2338,11 @@ struct page *__alloc_pages_exact(gfp_t g
> > 	page = alloc_pages(gfp_mask, order);
> >  	if (page) {
> > -		struct page *alloc_end = page + (1 << order);
> > -		struct page *used = page + nr_pages;
> > +		struct page *unused_start = page + nr_pages;
> > +		int nr_unused = (1 << order) - nr_pages;
> 
> How about unsigned long?

Personally, I'd rather leave this up to the poor sucker that tries to
set MAX_ORDER to 33.  If someone did that, we'd end up with kernels that
couldn't even boot on systems with less than 16GB of RAM since the
(required) flatmem mem_map[] would take up ~14.3GB.  They couldn't
handle memory holes and couldn't be NUMA-aware, either. 

So, if someone had a system like that, fixed up all the other spots
where we store numbers of pages in ints, and then did an 8TB+4k
allocation, yes, this would matter.  I'd rather save the 10 bytes of
source code and 4 bytes of stack than account for such an impossibly
improbable system.

-- Dave

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