Hi tejun, On 10/10/2013 03:20 AM, 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. > >> And I am not familiar with the ISA DMA limit, does this mean the memory >> below 4G? Just as we have the ZONE_DMA32 in x86_64. (16MB limit seems not >> the case here) > > Yeah, I was referring to the 16MB limit, which apparently ceased to > exist. Hmmmm...If we are talking 16MB limit hear, I don't think it a problem, either. Currently, default loading & running address of kernel is 16MB, so the kernel itself is above 16MB, memory allocated in bottom-up mode is obviously above the 16MB. Just seeing from a RHEL6.3 server: 01000000-01507ff4 : Kernel code 01507ff5-01c07b2f : Kernel data 01d4e000-02012023 : Kernel bss IOW, even if kernel is loaded and running at 1MB, it self will occupy about 16MB from the above. -- Thanks. Zhang Yanfei -- 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>