linux-kernel - Re: Cannot hot remove a memory device

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Date:	Mon, 12 Aug 2013 14:40:43 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	rafael.j.wysocki@...el.com, vasilis.liaskovitis@...fitbricks.com,
	linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
	tangchen@...fujitsu.com, wency@...fujitsu.com
Subject: Re: Cannot hot remove a memory device

On Sun, 2013-08-11 at 23:13 +0200, Rafael J. Wysocki wrote:
> On Thursday, August 08, 2013 04:50:42 PM Toshi Kani wrote:
> > On Fri, 2013-08-09 at 00:12 +0200, Rafael J. Wysocki wrote:
> > > On Thursday, August 08, 2013 11:15:20 AM Toshi Kani wrote:
> > > > On Fri, 2013-08-02 at 18:04 -0600, Toshi Kani wrote:
> > > > > 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:
> > > >  :
> > > > > > > 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.
> > > > 
> > > > FYI: I have submitted the patch blow to close this part of the issue...
> > > > 
> > > > https://lkml.org/lkml/2013/8/8/396
> > > 
> > > That looks good to me, but we'd still need to make it possible to have
> > > memory blocks smaller than 128 MB ...
> > 
> > Do you mean acpi_bind_one() needs to be able to handle such case?  If
> > so, it should not be a problem since a memory block device won't be
> > created when add_memory() fails with the change above.  So, there is no
> > binding to be done.  If you mean add_memory() needs to be able to handle
> > a smaller range, that's quite a tough one unless we make the section
> > size smaller.
> > 
> > BTW, when add_memory() fails, the memory hot-add request still succeeds
> > with no driver attached.  This seems logical, but the added device is
> > useless when no handler is attached.  And it does not allow ejecting the
> > device with no handler.  I am not too worry about this since this is a
> > rare case, but it reminded me that the framework won't handle rollback.
> 
> I'm not sure which rollback you mean.  During removal?

I meant rollback during hot-add.  Ideally, a device should be either
added in usable state (success) or failed back to the original state
(rollback).  Added in un-usable state is not really a success for users,
and creates an odd state to deal with.  But it is still a LOT better
than crashing the system.  So, I think this outcome is reasonable on
this framework because adding rollback at this point will complicate the
things unnecessarily.

> There are two slight problems here in my view.  First, even if the device
> cannot be ejected directly, it still will be removed when its parent is
> ejected, so it may be more consistent to just allow everything to be ejected
> regardless of whether or not it has a scan handler.  

Agreed.

> Second, I guess the
> removal is undesirable for memory devices for which the registration of the
> scan handler failed, so it would be good to fail the "offline" of such devices
> regardless of how we get there.  That's why I thought it would be good to have
> an "offline disabled" flag in struct acpi_device.

I see.  But when attach() failed, the memory device may not be used by
the kernel.  So, I think it should be safe to remove it.

Thanks,
-Toshi

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