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Re: [Xen-devel] [PATCH RFC] mm/memory_hotplug: Introduce memory block types

On Mon, 26 Nov 2018 16:59:14 +0100
David Hildenbrand <david@xxxxxxxxxx> wrote:

> On 26.11.18 15:20, Michal Suchánek wrote:
> > On Mon, 26 Nov 2018 14:33:29 +0100
> > David Hildenbrand <david@xxxxxxxxxx> wrote:
> >   
> >> On 26.11.18 13:30, David Hildenbrand wrote:  
> >>> On 23.11.18 19:06, Michal Suchánek wrote:    
> >   
> >>>>
> >>>> If we are going to fake the driver information we may as well add the
> >>>> type attribute and be done with it.
> >>>>
> >>>> I think the problem with the patch was more with the semantic than the
> >>>> attribute itself.
> >>>>
> >>>> What is normal, paravirtualized, and standby memory?
> >>>>
> >>>> I can understand DIMM device, baloon device, or whatever mechanism for
> >>>> adding memory you might have.
> >>>>
> >>>> I can understand "memory designated as standby by the cluster
> >>>> administrator".
> >>>>
> >>>> However, DIMM vs baloon is orthogonal to standby and should not be
> >>>> conflated into one property.
> >>>>
> >>>> paravirtualized means nothing at all in relationship to memory type and
> >>>> the desired online policy to me.    
> >>>
> >>> Right, so with whatever we come up, it should allow to make a decision
> >>> in user space about
> >>> - if memory is to be onlined automatically    
> >>
> >> And I will think about if we really should model standby memory. Maybe
> >> it is really better to have in user space something like (as Dan noted)  
> > 
> > If it is possible to designate the memory as standby or online in the
> > s390 admin interface and the kernel does have access to this
> > information it makes sense to forward it to userspace (as separate
> > s390-specific property). If not then you need to make some kind of
> > assumption like below and the user can tune the script according to
> > their usecase.  
> Also true, standby memory really represents a distinct type of memory
> block (memory seems to be there but really isn't). Right now I am
> thinking about something like this (tried to formulate it on a very
> generic level because we can't predict which mechanism might want to
> make use of these types in the future).
> /*
>  * Memory block types allow user space to formulate rules if and how to
>  * online memory blocks. The types are exposed to user space as text
>  * strings in sysfs. While the typical online strategies are described
>  * along with the types, there are use cases where that can differ (e.g.
>  * use MOVABLE zone for more reliable huge page usage, use NORMAL zone
>  * due to zone imbalance or because memory unplug is not intended).
>  *
>  *  No memory block is to be created (e.g. device memory). Used internally
>  *  only.
>  *
>  *  This memory block type should be treated as if it can be
>  *  removed/unplugged from the system again. E.g. there is a hardware
>  *  interface to unplug such memory. This memory block type is usually
>  *  onlined to the MOVABLE zone, to e.g. make offlining of it more
>  *  reliable. Examples include ACPI and PPC DIMMs.
>  *
>  *  This memory block type should be treated as if it can not be
>  *  removed/unplugged again. E.g. there is no hardware interface to
>  *  unplug such memory. This memory block type is usually onlined to
>  *  the NORMAL zone, as offlining is not beneficial. Examples include boot
>  *  memory on most architectures and memory added via balloon devices.

AFAIK baloon device can be inflated as well so this does not really
describe how this memory type works in any meaningful way. Also it
should not be possible to see this kind of memory from userspace. The
baloon driver just takes existing memory that is properly backed,
allocates it for itself, and allows the hypervisor to use it. Thus it
creates the equivalent to s390 standby memory which is not backed in
the VM. When memory is reclaimed from hypervisor the baloon driver
frees it making it available to the VM kernel again. However, the whole
time the memory appears present in the machine and no hotplug events
should be visible unless the docs I am looking at are really outdated.

>  *
>  *  The memory block type should be treated as if it can be
>  *  removed/unplugged again, however the actual memory hot(un)plug is
>  *  performed by onlining/offlining. In virtual environments, such memory
>  *  is usually added during boot and never removed. Onlining memory will
>  *  result in memory getting allocated to a VM. This memory type is usually
>  *  not onlined automatically but explicitly by the administrator. One
>  *  example is standby memory on s390x.

Again, this does not meaningfully describe the memory type. There is
no memory on standby. There is in fact no backing at all unless you
online it. So this probably is some kind of shared memory. However, the
(de)allocation is controlled differently compared to the baloon device.
The concept is very similar, though.



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