On Tue, 2007-05-15 at 13:40 -0400, Robert P. J. Day wrote: > from different sources, i seem to be reading conflicting > explanations about what happens with "high memory" on an x86 -- the > physical memory in the range 896M-1G. > > as i read it, the normal ranges are: > > ZONE_DMA: 0-16M > ZONE_NORMAL: 16M-896M > ZONE_HIGHMEM: 896M-1G > > now, what *exactly* does the kernel do with the 128M available in > ZONE_HIGHMEM? one source tells me that that area is reserved > exclusively for various kernel data structures, like memory map and > page table info. but is that *all* it's used for? > > certainly, if the kernel needs to access any physical memory beyond > 1G, it needs to be mapped into the kernel virtual address space first. > but must that memory be mapped *only* into ZONE_NORMAL, or could it > also be mapped into ZONE_HIGHMEM and share that area with the kernel > data structures if there's room? Physical memory beyond 896M is always mapped temporarily in the last 128M of kernel virtual memory(in case of 3G/1G split). This virtual memory size would be exactly equal to the total memory mapped by one page table(i.e. 1024 pte's). This is the window kernel has, in order to map/unmap >896M physical memory. Apart from this, the last 128M is majorly used as vmalloc area. > > rday > > p.s. where in the kernel is the beginning of ZONE_HIGHMEM defined > anyway? is it configurable? thanks for any clarification. You can certainly increase the kernel virtual memory size(and as an effect the ZONE_NORMAL size) by using a 2G/2G memory model. And, the ZONE_HIGHMEM start will always be at the end of ZONE_NORMAL. Which usually is decided by a variable defined in arch/i386/mm/init.c as unsigned int __VMALLOC_RESERVE = 128 << 20; cheers, sandeep -- To unsubscribe from this list: send an email with "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx Please read the FAQ at http://kernelnewbies.org/FAQ
