Subject: Re: [PATCH] KVM: PPC: BOOK3S: HV: Don't try to allocate from kernel page allocator for hash page table.

From: Alexander Graf <[email-masked]>
Date: Tue, 06 May 2014 09:05:42 +0200

On 06.05.14 02:06, Benjamin Herrenschmidt wrote:
> On Mon, 2014-05-05 at 17:16 +0200, Alexander Graf wrote:
>> Isn't this a greater problem? We should start swapping before we hit
>> the point where non movable kernel allocation fails, no?
> Possibly but the fact remains, this can be avoided by making sure that
> if we create a CMA reserve for KVM, then it uses it rather than using
> the rest of main memory for hash tables.

So why were we preferring non-CMA memory before? Considering that Aneesh 
introduced that logic in fa61a4e3 I suppose this was just a mistake?

>> The fact that KVM uses a good number of normal kernel pages is maybe
>> suboptimal, but shouldn't be a critical problem.
> The point is that we explicitly reserve those pages in CMA for use
> by KVM for that specific purpose, but the current code tries first
> to get them out of the normal pool.
>
> This is not an optimal behaviour and is what Aneesh patches are
> trying to fix.

I agree, and I agree that it's worth it to make better use of our 
resources. But we still shouldn't crash.

However, reading through this thread I think I've slowly grasped what 
the problem is. The hugetlbfs size calculation.

I guess something in your stack overreserves huge pages because it 
doesn't account for the fact that some part of system memory is already 
reserved for CMA.

So the underlying problem is something completely orthogonal. The patch 
body as is is fine, but the patch description should simply say that we 
should prefer the CMA region because it's already reserved for us for 
this purpose and we make better use of our available resources that way.

All the bits about pinning, numa, libvirt and whatnot don't really 
matter and are just details that led Aneesh to find this non-optimal 
allocation.

Alex