On Tue, 22 Apr 2014 17:37:26 -0400 Luiz Capitulino <lcapitulino@xxxxxxxxxx> wrote: > On Thu, 17 Apr 2014 16:01:10 -0700 > Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > > On Thu, 10 Apr 2014 13:58:40 -0400 Luiz Capitulino <lcapitulino@xxxxxxxxxx> 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. afacit we'd need to increase SECTION_SIZE_BITS to 29 or more to accommodate 1G MAX_ORDER. I assume this means that some machines with sparse physical memory layout may not be able to use all (or as much) of the physical memory. Perhaps Yinghai can advise? I do think we should fully explore this option before giving up and adding new special-case code. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>