RE: [Qemu-devel] [PATCH kernel v5 0/5] Extend virtio-balloon for fast (de)inflating & fast live migration

[Li, Liang Z]

> On Fri, Dec 16, 2016 at 01:12:21AM +0000, Li, Liang Z wrote:
> > There still exist the case if the MAX_ORDER is configured to a large
> > value, e.g. 36 for a system with huge amount of memory, then there is only
> 28 bits left for the pfn, which is not enough.
> 
> Not related to the balloon but how would it help to set MAX_ORDER to 36?
> 

My point here is  MAX_ORDER may be configured to a big value.

> What the MAX_ORDER affects is that you won't be able to ask the kernel
> page allocator for contiguous memory bigger than 1<<(MAX_ORDER-1), but
> that's a driver issue not relevant to the amount of RAM. Drivers won't
> suddenly start to ask the kernel allocator to allocate compound pages at
> orders >= 11 just because more RAM was added.
> 
> The higher the MAX_ORDER the slower the kernel runs simply so the smaller
> the MAX_ORDER the better.
> 
> > Should  we limit the MAX_ORDER? I don't think so.
> 
> We shouldn't strictly depend on MAX_ORDER value but it's mostly limited
> already even if configurable at build time.
> 

I didn't know that and will take a look, thanks for your information.


Liang
> We definitely need it to reach at least the hugepage size, then it's mostly
> driver issue, but drivers requiring large contiguous allocations should rely on
> CMA only or vmalloc if they only require it virtually contiguous, and not rely
> on larger MAX_ORDER that would slowdown all kernel allocations/freeing.