Re: [Bug 25042] New: RAM buffer I/O resource badly interacts with memory hot-add

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Tue, 4 Jan 2011 13:51:48 -0800
Andrew Morton <akpm@linux-foundation.org> wrote:

> 
> (switched to email.  Please respond via emailed reply-to-all, not via the
> bugzilla web interface).
> 
> I'm not sure who to blame here so I'll just spray it at everyone I've
> ever met ;)
> 
> On Thu, 16 Dec 2010 23:00:12 GMT
> bugzilla-daemon@bugzilla.kernel.org wrote:
> 
> > https://bugzilla.kernel.org/show_bug.cgi?id=25042
> > 
> >            Summary: RAM buffer I/O resource badly interacts with memory
> >                     hot-add
> >            Product: Memory Management
> >            Version: 2.5
> >     Kernel Version: 2.6.35
> >           Platform: All
> >         OS/Version: Linux
> >               Tree: Mainline
> >             Status: NEW
> >           Severity: normal
> >           Priority: P1
> >          Component: Other
> >         AssignedTo: akpm@linux-foundation.org
> >         ReportedBy: petr@vandrovec.name
> >                 CC: akataria@vmware.com
> >         Regression: Yes
> > 
> > 
> > Created an attachment (id=40502)
> >  --> (https://bugzilla.kernel.org/attachment.cgi?id=40502)
> > /proc/iomem after issuing hot-add, one from 3076 to 3200, other from 3200 to
> > 3456MB
> > 
> > Linus's commit 45fbe3ee01b8e463b28c2751b5dcc0cbdc142d90 in May 2009 added code
> > to create 'RAM buffer' above top of RAM to ensure that I/O resources do not
> > start immediately after RAM, but sometime later.  Originally it was enforcing
> > 32MB alignment, now it enforces 64MB.  Which means that in VMs with memory size
> > which is not multiple of 64MB there will be additional 'RAM buffer' resource
> > present:
> > 
> > 100000000-1003fffff : System RAM
> > 100400000-103ffffff : RAM buffer
> > 
> > When we try to hot-add memory, kernel complains that there was resource
> > conflict with this fake 'RAM buffer' and hot-added memory is not recognized:
> > 
> > [  115.324952] Hotplug Mem Device 
> > [  115.325549] System RAM resource 100400000 - 10fffffff cannot be added
> > [  115.325553] ACPI:memory_hp:add_memory failed
> > [  115.326519] ACPI:memory_hp:Error in acpi_memory_enable_device
> > [  115.327183] acpi_memhotplug: probe of PNP0C80:00 failed with error -22
> > [  115.327347] 
> > [  115.327350]  driver data not found
> > [  115.328808] ACPI:memory_hp:Cannot find driver data
> > 
> > For now we've modified hotplug code to split hot-added request into smaller
> > ranges, so only first <= 252MB are unusable, rather than whole xxxGB chunk, but
> > if 'RAM buffer' could be made dependent on memory hot-plug not available on the
> > platform, it would be much better.
> > 

Hmm ? Why do you need to place "hot-added" memory's address range next to System
RAM ? Sparsemem allows sparse memory layout.

Is it very difficult ?


Thanks,
-Kame

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