Re: [PATCH 2/2 v2, RFC] Driver core: Introduce offline/online callbacks for memory blocks

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Monday, May 06, 2013 06:28:12 PM Vasilis Liaskovitis wrote:
> Hi,
> 
> On Sat, May 04, 2013 at 01:21:16PM +0200, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > 
> > Introduce .offline() and .online() callbacks for memory_subsys
> > that will allow the generic device_offline() and device_online()
> > to be used with device objects representing memory blocks.  That,
> > in turn, allows the ACPI subsystem to use device_offline() to put
> > removable memory blocks offline, if possible, before removing
> > memory modules holding them.
> > 
> > The 'online' sysfs attribute of memory block devices will attempt to
> > put them offline if 0 is written to it and will attempt to apply the
> > previously used online type when onlining them (i.e. when 1 is
> > written to it).
> > 
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > ---
> >  drivers/base/memory.c  |  105 +++++++++++++++++++++++++++++++++++++------------
> >  include/linux/memory.h |    1 
> >  2 files changed, 81 insertions(+), 25 deletions(-)
> >
> [...]
> 
> > @@ -686,10 +735,16 @@ int offline_memory_block(struct memory_b
> >  {
> >  	int ret = 0;
> >  
> > +	lock_device_hotplug();
> >  	mutex_lock(&mem->state_mutex);
> > -	if (mem->state != MEM_OFFLINE)
> > -		ret = __memory_block_change_state(mem, MEM_OFFLINE, MEM_ONLINE, -1);
> > +	if (mem->state != MEM_OFFLINE) {
> > +		ret = __memory_block_change_state_uevent(mem, MEM_OFFLINE,
> > +							 MEM_ONLINE, -1);
> > +		if (!ret)
> > +			mem->dev.offline = true;
> > +	}
> >  	mutex_unlock(&mem->state_mutex);
> > +	unlock_device_hotplug();
> 
> (Testing with qemu...)

Thanks!

> offline_memory_block is called from remove_memory, which in turn is called from
> acpi_memory_device_remove (detach operation) during acpi_bus_trim. We already
> hold the device_hotplug lock when we trim (acpi_scan_hot_remove), so we
> don't need to lock/unlock_device_hotplug in offline_memory_block.

Indeed.

First, it looks like offline_memory_block_cb() is the only place calling
offline_memory_block(), is that right?  I'm wondering if it would make
sense to use device_offline() in there and remove offline_memory_block()
entirely?

Second, if you ran into this issue during testing, that would mean that patch
[1/2] actually worked for you, which would be nice. :-)  Was that really the
case?

> A more general issue is that there are now two memory offlining efforts:
> 
> 1) from acpi_bus_offline_companions during device offline
> 2) from mm: remove_memory during device detach (offline_memory_block_cb)
> 
> The 2nd is only called if the device offline operation was already succesful, so
> it seems ineffective or redundant now, at least for x86_64/acpi_memhotplug machine
> (unless the blocks were re-onlined in between).

Sure, and that should be OK for now.  Changing the detach behavior is not
essential from the patch [2/2] perspective, we can do it later.

> On the other hand, the 2nd effort has some more intelligence in offlining, as it
> tries to offline twice in the precense of memcg, see commits df3e1b91 or
> reworked 0baeab16. Maybe we need to consolidate the logic.

Hmm.  Perhaps it would make sense to implement that logic in
memory_subsys_offline(), then?

> remove_memory is called from device_detach, during trim that can't fail, so it
> should not fail. However this function can still fail in 2 cases:
> - offline_memory_block_cb
> - is_memblock_offlined_cb
> in the case of re-onlined memblocks in between device-offline and device detach.
> This seems possible I think, since we do not hold lock_memory_hotplug for the
> duration of the hot-remove operation.

But we do hold device_hotplug_lock, so every code path that may race with
acpi_scan_hot_remove() needs to take device_hotplug_lock as well.  Now,
question is whether or not there are any code paths like that calling one of
the two functions above without holding device_hotplug_lock?

Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.

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