questions about memory hotplug

[Larry Bassel]

On 30 Jul 11 17:30, Shaohua Li wrote:
> On Sat, Jul 30, 2011 at 06:12:31AM +0800, Larry Bassel wrote:
> > We want to handle the following 2 use cases we have on
> > some of our (ARM) platforms:
> > 
> > 1. A platform where part of the memory may be powered
> > off. The location and size of this memory is not known
> > until the kernel parses the memory tags (there aren't
> > any non-standard tags used, but the memory layout and
> > a memory bank size which is obtained from HW are used
> > to figure out where and how large this memory is). All
> > of this memory must either be on or off. For a given
> > configuration the location of this memory can't be moved.
> > 
> > 2. A (different) platform where part of the memory
> > is occasionally needed as a large physically contiguous 
> > block, but usually is not (and then should be usable
> > by the kernel as normal memory). When the memory is
> > needed for the contiguous block, the pages in this
> > range currently being used by the kernel will
> > need to be migrated out.
> > 
> > The size of this is known at compile time and can be
> > placed at any reasonable place in memory (but should
> > be SPARSEMEM section aligned -- its size may however
> > not be a power of 2 and thus this memory could
> > span more than one section). This memory will not be
> > powered off, but must either be completely used for
> > one purpose or the other.
> > 
> > The size of memory in question for #1 is generally
> > much larger than that of #2.
> > 
> > Memory hotplug/hotremove (logical and physical for #1,
> > only logical for #2) approximately solves these problems,
> > but there is some functionality we need that AFAIK is
> > not present:
> > 
> > * The memory in #1 and #2 above must be in a movable
> > zone so that the chance of migration is maximized.
> > I'm familiar with the kernelcore= and movablecore=
> > commandline options, but they don't do what is necessary
> > here, because we need control on where the movable zone
> > is formed as well as the size. Also the location and size
> > of these special memory areas is not known until the
> > kernel comes up (in #1 it is conceivable that the bootloader
> > could locate the memory that can be powered on and off and
> > pass it in via some commandline option, but AFAICT this won't
> > work at all for #2).
> > 
> > One could hack up find_zone_movable_pfns_for_nodes() presumably,
> > but I wonder if there is an already existing way of doing
> > this (or at least a clean extension to the current
> > functionality that someone might suggest).
> > 
> > Would CONFIG_ARCH_POPULATES_NODE_MAP help here? Does anyone
> > use this? It doesn't seem to be in any defconfig or Kconfig
> > on 3.0 (or earlier versions I've looked at).
> > 
> > Perhaps CMA (which is not merged yet and AFAIK still has some
> > issues on ARM) might handle #2 better than memory
> > hotplug/hotremove. Or is there a better way to handle #2
> > than either CMA or memory hotplug?
> is such memory always at the end of memory? if yes, such memory
> can be added to MOVABLE_ZONE.

In use case #1 yes, maybe not in #2 (we can arrange it to be
at the end of memory, but then might waste memory as it may
not be aligned on a SPARSEMEM section boundary and so would
need to be padded).

> or if such memory can be at the middle of memory, maybe we can add
> a new mirgate type, the migrate type is movable but other type allocation
> can't fallback into.
> 
> Thanks,
> Shaohua

Larry

> 
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