Re: [PATCH] Add __GFP_MOVABLE for callers to flag allocations that may be migrated

On Thu, 7 Dec 2006, Jeremy Fitzhardinge wrote:

> You can also deal with memory hotplug by adding a Xen-style
> pseudo-physical vs machine address abstraction.  This doesn't help with
> making space for contiguous allocations, but it does allow you to move
> "physical" pages from one machine page to another if you want to.  The
> paravirt ops infrastructure has already appeared in -git, and I'll soon
> have patches to allow Xen's paravirtualized mmu mode to work with it,
> which is a superset of what would be required to implement movable pages
> for hotpluggable memory.
> 
> (I don't know if you actually want to consider this approach; I'm just
> pointing out that it definitely a bad idea to conflate the two problems
> of memory fragmentation and hotplug.)

The same can be done using the virtual->physical mappings that exist on 
many platforms for the kernel address space (ia64 dynamically calculates 
those, x86_64 uses a page table with 2M pages for mapping the kernel). The 
problem is that the 1-1 mapping between physical and virtual addresses 
will have to be (at least partially) sacrificed which may lead to 
complications with DMA devices.


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