Date: Fri, 4 Aug 2006 12:13:08 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
To: kmannth@us.ibm.com
Cc: linux-kernel@vger.kernel.org, lhms-devel@lists.sourceforge.net,
       y-goto@jp.fujitsu.com, akpm@osdl.org
Subject: Re: [PATCH] memory hotadd fixes [4/5] avoid check in acpi
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On Thu, 03 Aug 2006 20:00:08 -0700
keith mannthey <kmannth@us.ibm.com> wrote:


> > >   What protecting is there for calling add_memory on an already present
> > > memory range?  
> > > 
> > For example, considering ia64, which has 1Gbytes section...
> 
> Maybe 1gb sections is too large?  
> 
ia64 machines sometimes to have crazy big memory...so 1gb section is requested.
Configurable section_size for small machines was rejected in old days.


> > hot add following region.
> > ==
> > (A) 0xc0000000 - 0xd7ffffff  (section 3)
> > (B) 0xe0000000 - 0xffffffff  (section 3)
> > ==
> > (A) and (B) will go to the same section, but there is a memory hole between
> > (A) and (B). Considering memory (B) appears after (A) in DSDT.
> > 
> > After add_memory() against (A) is called, section 3 is ready.
> > Then, pfn_valid(0xe0000000) and pfn_valid(0xffffffff) returns true because
> > they are in section 3.
> > So, checking pfn_valid() for (B) will returns true and memory (B) cannot be
> > added. ioresouce collision check will help this situation.
> 
> With iommus out there throwing aliment all off way the flexability is
> good. 
> 
> My question is this.
> 
> Assuming 0-0xbfffffff is present.
> 
> What keeps 0xa0000000 to 0xa1000000 from being re-onlined by a bad call
> to add_memory?

Usual sparsemem's add_memory() checks whether there are sections in
sparse_add_one_section(). then add_pages() returns -EEXIST (nothing to do).
And ioresouce collision check will finally find collision because 0-0xbffffff
resource will conflict with 0xa0000000 to 0xa10000000 area.
But, x86_64 's (not sparsemem) add_pages() doen't do collision check, so it panics.
I posted patch to catch collision before calling arch_add_memory(), I think it will
help x86_64 users. and no-re-online.

-Kame


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