On Wed, 2013-10-09 at 15:20 -0400, Tejun Heo wrote: > Hello, > > On Thu, Oct 10, 2013 at 01:14:23AM +0800, Zhang Yanfei wrote: > > >> You meant that the memory size is about few megs. But here, page tables > > >> seems to be large enough in big memory machines, so that page tables will > > > > > > Hmmm? Even with 4k mappings and, say, 16Gigs of memory, it's still > > > somewhere above 32MiB, right? And, these physical mappings don't > > > usually use 4k mappings to begin with. Unless we're worrying about > > > ISA DMA limit, I don't think it'd be problematic. > > > > I think Peter meant very huge memory machines, say 2T memory? In the worst > > case, this may need 2G memory for page tables, seems huge.... > > Realistically tho, why would people be using 4k mappings on 2T > machines? For the sake of argument, let's say 4k mappings are > required for some weird reason, even then, doing SRAT parsing early > doesn't necessarily solve the problem in itself. It'd still need > heuristics to avoid occupying too much of 32bit memory because it > isn't difficult to imagine specific NUMA settings which would drive > page table allocation into low address. > > No matter what we do, there's no way around the fact that this whole > effort is mostly an incomplete solution in its nature and that's why I > think we better keep things isolated and simple. It isn't a good idea > to make structural changes to accomodate something which isn't and > doesn't have much chance of becoming a full solution. In addition, > the problem itself is niche to begin with. Let's not assume that memory hotplug is always a niche feature for huge & special systems. It may be a niche to begin with, but it could be supported on VMs, which allows anyone to use. Vasilis has been working on KVM to support memory hotplug. Thanks, -Toshi -- 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>