Re: [PATCH 0/3] KVM: PPC: Book3S HV: More flexible allocator for linear memory

[Alexander Graf <agraf@suse.de>]

On 14.09.2012, at 10:11, Paul Mackerras wrote:

> On Fri, Sep 14, 2012 at 01:32:23AM +0200, Alexander Graf wrote:
>> 
>> On 12.09.2012, at 02:34, Paul Mackerras wrote:
>> 
>>> This series of 3 patches makes it possible for guests to allocate
>>> whatever size of HPT they need from linear memory preallocated at
>>> boot, rather than being restricted to a single size of HPT (by
>>> default, 16MB) and having to use the kernel page allocator for
>>> anything else -- which in practice limits them to at most 16MB given
>>> the default value for the maximum page order.  Instead of allocating
>>> many individual pieces of memory, this allocates a single contiguous
>>> area and uses a simple bitmap-based allocator to hand out pieces of it
>>> as required.
>> 
>> Have you tried to play with CMA for this? It sounds like it could buy us exactly what we need.
> 
> Interesting, I hadn't noticed that there.  I had a bit of a look at
> it, and it's certainly in the right general direction, however it
> would need some changes to do what we need.  It limits the alignment
> to at most 512 pages, i.e. 2MB with 4k pages or 32MB with 64k pages,
> but we need RMAs of 64MB to 256MB for PPC970 and they have to be
> aligned on their size, as do the HPTs for PPC970.
> 
> Secondly, it has a link with the page allocator that I don't fully
> understand, but it seems from the comments in alloc_contig_range()
> (mm/page_alloc.c) that you can allocate at most MAX_ORDER_NR_PAGES
> pages at once, and that defaults to 16MB for ppc64, which isn't nearly
> enough.  If that's true then it would make it unusable for this.

So do you think it makes more sense to reimplement a large page allocator in KVM, as this patch set does, or improve CMA to get us really big chunks of linear memory?

Let's ask the Linux mm guys too :). Maybe they have an idea.


Alex

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