Re: [PATCH v3 0/5] hugetlb: add support gigantic page allocation at runtime

[Luiz Capitulino <lcapitulino@redhat.com>]

On Thu, 17 Apr 2014 16:01:10 -0700
Andrew Morton <akpm@linux-foundation.org> wrote:

> On Thu, 10 Apr 2014 13:58:40 -0400 Luiz Capitulino <lcapitulino@redhat.com> wrote:
> 
> > The HugeTLB subsystem uses the buddy allocator to allocate hugepages during
> > runtime. This means that hugepages allocation during runtime is limited to
> > MAX_ORDER order. For archs supporting gigantic pages (that is, page sizes
> > greater than MAX_ORDER), this in turn means that those pages can't be
> > allocated at runtime.
> 
> Dumb question: what's wrong with just increasing MAX_ORDER?

To be honest I'm not a buddy allocator expert and I'm not familiar with
what is involved in increasing MAX_ORDER. What I do know though is that it's
not just a matter of increasing a macro's value. For example, for sparsemem
support we have this check (include/linux/mmzone.h:1084):

#if (MAX_ORDER - 1 + PAGE_SHIFT) > SECTION_SIZE_BITS
#error Allocator MAX_ORDER exceeds SECTION_SIZE
#endif

I _guess_ it's because we can't allocate more pages than what's within a
section on sparsemem. Can sparsemem and the other stuff be changed to
accommodate a bigger MAX_ORDER? I don't know. Is it worth it to increase
MAX_ORDER and do all the required changes, given that a bigger MAX_ORDER is
only useful for HugeTLB and the archs supporting gigantic pages? I'd guess not.