linux-kernel - Re: [patch]x86: arch_add

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Date:	Wed, 8 Oct 2008 23:40:34 -0700
From:	"Yinghai Lu" <yinghai@...nel.org>
To:	"Shaohua Li" <shaohua.li@...el.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Ingo Molnar" <mingo@...hat.com>
Subject: Re: [patch]x86: arch_add_memory round up address

On Wed, Oct 8, 2008 at 11:28 PM, Shaohua Li <shaohua.li@...el.com> wrote:
> On Thu, Oct 09, 2008 at 02:22:50PM +0800, Yinghai Lu wrote:
>> On Wed, Oct 8, 2008 at 11:08 PM, Shaohua Li <shaohua.li@...el.com> wrote:
>> > On Thu, 2008-10-09 at 14:04 +0800, Yinghai Lu wrote:
>> >> On Wed, Oct 8, 2008 at 10:31 PM, Shaohua Li <shaohua.li@...el.com> wrote:
>> >> > Round up address to a page, otherwise the last page isn't mapped.
>> >> >
>> >> > Signed-off-by: Shaohua Li <shaohua.li@...el.com>
>> >> > ---
>> >> >  arch/x86/mm/init_64.c |    3 ++-
>> >> >  1 file changed, 2 insertions(+), 1 deletion(-)
>> >> >
>> >> > Index: linux/arch/x86/mm/init_64.c
>> >> > ===================================================================
>> >> > --- linux.orig/arch/x86/mm/init_64.c    2008-10-09 11:42:33.000000000 +0800
>> >> > +++ linux/arch/x86/mm/init_64.c 2008-10-09 11:43:22.000000000 +0800
>> >> > @@ -721,7 +721,8 @@ int arch_add_memory(int nid, u64 start,
>> >> >        unsigned long nr_pages = size >> PAGE_SHIFT;
>> >> >        int ret;
>> >> >
>> >> > -       last_mapped_pfn = init_memory_mapping(start, start + size-1);
>> >> > +       last_mapped_pfn = init_memory_mapping(start,
>> >> > +               round_up(start + size-1, PAGE_SIZE));
>> >> >        if (last_mapped_pfn > max_pfn_mapped)
>> >> >                max_pfn_mapped = last_mapped_pfn;
>> >>
>> >> should use
>> >>
>> >> last_mapped_pfn = init_memory_mapping(start, start + size);
>> > No, this still can't guarantee page aligned, though this works in my
>> > test
>>
>> who will call arch_add_memory? that should be start and size already
>> be page aligned.
> It's memory hotplug. Doing a round up is always ok and safe even it might be already aligned.
>

it seems rounding up in that case is wrong...

if the caller call that funtion with extra half page, you don't need
map that half page, because you can not use it.

YH
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