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

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Message-ID: <20081014073011.GA1556@sli10-desk.sh.intel.com>
Date:	Tue, 14 Oct 2008 15:30:11 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Yasunori Goto <y-goto@...fujitsu.com>
Cc:	Yinghai Lu <yinghai@...nel.org>,
	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 Tue, Oct 14, 2008 at 01:16:46PM +0800, Yasunori Goto wrote:
> > On Thu, Oct 09, 2008 at 02:40:34PM +0800, Yinghai Lu wrote:
> > > 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.
> > shouldn't we mark such page as reserved and so it will not be used? This is the way we handle hole.
> 
> Just curious,
> have you ever seen a real machine which needs this patch?
No, I just did some experiments on a desktop for memory hotplug and this bug
triggered a crash in my test.
Yinghai's suggestion also fixed the bug. I just want to have safer method. Anyway, either approach is ok to me.

Thanks,
Shaohua
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