Re: Cannot hot remove a memory device

[Toshi Kani <toshi.kani@hp.com>]

On Sat, 2013-08-03 at 01:43 +0200, Rafael J. Wysocki wrote:
> On Friday, August 02, 2013 03:46:15 PM Toshi Kani wrote:
> > On Thu, 2013-08-01 at 23:43 +0200, Rafael J. Wysocki wrote:
> > > Hi,
> > > 
> > > Thanks for your report.
> > > 
> > > On Thursday, August 01, 2013 05:37:21 PM Yasuaki Ishimatsu wrote:
> > > > By following commit, I cannot hot remove a memory device.
> > > > 
> > > > ACPI / memhotplug: Bind removable memory blocks to ACPI device nodes
> > > > commit e2ff39400d81233374e780b133496a2296643d7d
> > > > 
> > > > Details are follows:
> > > > When I add a memory device, acpi_memory_enable_device() always fails
> > > > as follows:
> > > > 
> > > > ...
> > > > [ 1271.114116]  [ffffea121c400000-ffffea121c7fffff] PMD -> [ffff880813c00000-ffff880813ffffff] on node 3
> > > > [ 1271.128682]  [ffffea121c800000-ffffea121cbfffff] PMD -> [ffff880813800000-ffff880813bfffff] on node 3
> > > > [ 1271.143298]  [ffffea121cc00000-ffffea121cffffff] PMD -> [ffff880813000000-ffff8808133fffff] on node 3
> > > > [ 1271.157799]  [ffffea121d000000-ffffea121d3fffff] PMD -> [ffff880812c00000-ffff880812ffffff] on node 3
> > > > [ 1271.172341]  [ffffea121d400000-ffffea121d7fffff] PMD -> [ffff880812800000-ffff880812bfffff] on node 3
> > > > [ 1271.186872]  [ffffea121d800000-ffffea121dbfffff] PMD -> [ffff880812400000-ffff8808127fffff] on node 3
> > > > [ 1271.201481]  [ffffea121dc00000-ffffea121dffffff] PMD -> [ffff880812000000-ffff8808123fffff] on node 3
> > > > [ 1271.216041]  [ffffea121e000000-ffffea121e3fffff] PMD -> [ffff880811c00000-ffff880811ffffff] on node 3
> > > > [ 1271.230623]  [ffffea121e400000-ffffea121e7fffff] PMD -> [ffff880811800000-ffff880811bfffff] on node 3
> > > > [ 1271.245148]  [ffffea121e800000-ffffea121ebfffff] PMD -> [ffff880811400000-ffff8808117fffff] on node 3
> > > > [ 1271.259683]  [ffffea121ec00000-ffffea121effffff] PMD -> [ffff880811000000-ffff8808113fffff] on node 3
> > > > [ 1271.274194]  [ffffea121f000000-ffffea121f3fffff] PMD -> [ffff880810c00000-ffff880810ffffff] on node 3
> > > > [ 1271.288764]  [ffffea121f400000-ffffea121f7fffff] PMD -> [ffff880810800000-ffff880810bfffff] on node 3
> > 
> > It appears that each memory object only has 64MB of memory.  This is
> > less than the memory block size, which is 128MB.  This means that a
> > single memory block associates with two ACPI memory device objects.
> 
> That'd be bad.
> 
> How did that work before if that indeed is the case?

Well, it looks to me that it has never worked before...

> > > > ...	
> > > > [ 1271.325841] acpi PNP0C80:03: acpi_memory_enable_device() error
> > > 
> > > Well, the only new way acpi_memory_enable_device() can fail after that commit
> > > is a failure in acpi_bind_memory_blocks().
> > 
> > Agreed.
> > 
> > > This means that either handle is NULL, which I think we can exclude, because
> > > acpi_memory_enable_device() wouldn't be called at all if that were the case, or
> > > there's a more subtle error in acpi_bind_one().
> > > 
> > > One that comes to mind is that we may be calling acpi_bind_one() twice for the
> > > same memory region, in which it will trigger -EINVAL from the sanity check in
> > > there.
> > 
> > I think it fails with -EINVAL at the place with dev_warn(dev, "ACPI
> > handle is already set\n").  When two ACPI memory objects associate with
> > a same memory block, the bind procedure of the 2nd ACPI memory object
> > sees that ACPI_HANDLE(dev) is already set to the 1st ACPI memory object.
> 
> That sound's plausible, but I wonder how we can fix that?
> 
> There's no way for a single physical device to have two different ACPI
> "companions".  It looks like the memory blocks should be 64 M each in that
> case.  Or we need to create two child devices for each memory block and
> associate each of them with an ACPI object.  That would lead to complications
> in the user space interface, though.

Right.  Even bigger issue is that I do not think __add_pages() and
__remove_pages() can add / delete a memory chunk that is less than
128MB.  128MB is the granularity of them.  So, we may just have to fail
this case gracefully.

Thanks,
-Toshi