I think for some architectures, the position of highmem is constrained by hardware as well. It is not always a kernel decision and not always configurable as in case of x86.
In case of MIPS32, low memory is between 0 and 512 MB and high memory starts above 512 MB. Also the user space is of size 2 GB. Please see the definition of macros PAGE_OFFSET and HIGHMEM_START at : http://lxr.linux.no/linux+v2.6.33/arch/mips/include/asm/mach-generic/spaces.h This is because MIPS32 processors have KSEG0 and KSEG1 segments lying between 0 and 512 MB and KSEG2/3 lies above it. May be someone on the group can confirm this. ~Himanshu On Wed, Apr 7, 2010 at 6:20 PM, Chetan Nanda <chetanna...@gmail.com> wrote: > > > On Wed, Apr 7, 2010 at 5:40 PM, Xianghua Xiao <xiaoxiang...@gmail.com>wrote: > >> On Wed, Apr 7, 2010 at 12:48 AM, Venkatram Tummala >> <venkatram...@gmail.com> wrote: >> > I completely agree with you. I was just trying to clarify Xianghua's >> > statement "last 128 MB is used for HIGHMEM". I got the feeling that he >> > thought that last 128MB can be used for vmalloc, IO and for HIGHMEM. So, >> i >> > was clarifying that last 128MB is not "used for highmem" but it is used >> to >> > support highmem.(among many other things). That was what i intended. >> > >> > On Tue, Apr 6, 2010 at 7:09 PM, H. Peter Anvin <h...@zytor.com> wrote: >> >> >> >> On 04/06/2010 07:04 PM, Venkatram Tummala wrote: >> >> > Hey Xiao, >> >> > >> >> > last 128MB is not used for highmem. last 128MB is used for data >> >> > structures(page tables etc.) to support highmem . Highmem is not >> >> > something which is "INSIDE" Kernel's Virtual Address space. Highmem >> >> > refers to a region of "Physical memory" which can be mapped into >> >> > kernel's virtual address space through page tables. >> >> > >> >> > Regards, >> >> > Venkatram Tummala >> >> > >> >> >> >> Not quite. >> >> >> >> The vmalloc region is for *anything which is dynamically mapped*, which >> >> includes I/O, vmalloc, and HIGHMEM (kmap). >> >> >> >> -hpa >> >> >> >> -- >> >> H. Peter Anvin, Intel Open Source Technology Center >> >> I work for Intel. I don't speak on their behalf. >> >> >> > >> > >> >> Thanks Venkatram, do these sound right: >> >> 1. All HIGHMEM(physical address beyond 896MB) are kmapped back into >> the last 128MB kernel "virtual" address space(using page tables stored >> in the last 128MB physical address). That also implies it's a very >> limited virtual space for large memory system and need do kunmap when >> you're done with it(so you can kmap other physical memories in). >> I'm not familiar with large-memory systems, not sure how kmap cope >> with that using this limited 128M window assuming kernel is 1:3 split. >> > > Small correction here, page-table is not store in last 128MB of physical > memory. Instead the complete page table for 4GB virtual address space is > stored in kernel data-structures (kernel data section). > > ~cnanda > > 2. The last 128MB physical address can be used for page tables(kmap), >> vmalloc, IO,etc >> >> Regards, >> Xianghua >> >> -- >> To unsubscribe from this list: send an email with >> "unsubscribe kernelnewbies" to ecar...@nl.linux.org >> Please read the FAQ at http://kernelnewbies.org/FAQ >> >> >
