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

On Fri, Dec 16, 2016 at 01:12:21AM +0000, Li, Liang Z wrote:

quoted

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.

quoted

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.

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>