Messages in this thread | | | Date | Tue, 6 Apr 2010 12:20:22 -0700 | Subject | Re: why choose 896MB to the start point of ZONE_HIGHMEM | From | Frank Hu <> |
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On Tue, Apr 6, 2010 at 11:17 AM, H. Peter Anvin <hpa@zytor.com> wrote: > On 04/06/2010 08:02 AM, Joel Fernandes wrote: >> Hi Hayfeng, >> >> On Tue, Apr 6, 2010 at 8:07 PM, hayfeng Lee <teklife.kernel@gmail.com> wrote: >>> hello,every one. >>> I have a question: >>> Why does linux choose 896MB to do a start point of ZONE_HIGHMEM and >>> the end point of ZONE_NORMAL. Just for experience? >>> What is the advantages? >> >> This is not an advantage but a limitation of 32 bit processor and >> architecture. Only physical memory in first 896MB is directly mapped >> to the kernel virtual memory address space. This is called >> ZONE_NORMAL. To access any physical memory in ZONE_HIGHMEM, the kernel >> has to set up page table entries to indirectly map the physical memory >> into a virtual memory address (I think around 128MB or so worth page >> table entries are reused for this purpose). On the other hand, on 64 >> bit architectures, the entire physical memory is directly mapped and >> accessible to the kernel. ZONE_HIGHMEM doesn't exist on 64 bit. >> >> Take the above with a grain of salt, someone with a better knowledge >> about this intrusive topic can be give a more detailed explanation :) >> > > The ELF ABI specifies that user space has 3 GB available to it. That > leaves 1 GB for the kernel. The kernel, by default, uses 128 MB for I/O > mapping, vmalloc, and kmap support, which leaves 896 MB for LOWMEM. > > All of these boundaries are configurable; with PAE enabled the user > space boundary has to be on a 1 GB boundary. > > -hpa > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ >
the VM split is also configurable when building the kernel (for 32-bit processors). -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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