On Tue, 2007-04-17 at 20:51 +0530, Sandeep Sanjay Patil wrote: > On Tue, 2007-04-17 at 19:00 +0530, Prasad Joshi wrote: > > Hi All, > > > > I am confused with the ZONEs and HIGH memory. > > > > > > Memory within ZONE_NORMAL is directly mapped by the kernel into the > > upper region of the linear address space. This is the physical memory > > directly accessible by the kernel and is related to the > > kernel/userspace split in the linear address space marked by > > PAGE_OFFSET. > > > > > > Why ZONE_NORMAL is directly mapped into upper region of the linear > > address space? I think here PAGE_OFFSET (3GB) indicates the upper > > region of the linear address space ? > The starting address(which is 0 most of the time) of the physical memory is mapped to 0xc0000000 ie from zone_dma. > So that the kernel can _always_ access mem within ZONE_NORMAL. So, there > is a possibility that a page in ZONE_NORMAL is mapped in userspace to > (allocated to a process). So, then that page frame then has two mappings > one for the process and the other for the kernel itself. > > > > > > > ZONE_HIGHMEM is the remaining available memory in the system and is > > not directly mapped by the kernel. > > > > > > Generally, > > ZONE_DMA First 16MiB of memory > > ZONE_NORMAL 16MiB – 896MiB > > ZONE_HIGHMEM 896 MiB – End > > > > Now, the previous para says ZONE_NORMAL is at PAGE_OFFSET (3GB) but, > > this table indicates it at 16 MB ? > > > > Because the address space usable by the kernel (ZONE_NORMAL) is > > limited in size, the kernel has support for the concept of high > > memory. Physical memory is divided into zones(not virtual memory) so the address space usable by the kernel is virtual address 0xc0000000 to 0xffffffff , which maps directly from phy address 0. > Two thresholds of high memory exist on 32-bit x86 systems, one > > at 4GiB and a second at 64GiB. The 4GiB limit is related to the amount > > of memory that may be addressed by a 32-bit physical address. To > > access memory between the range of 1GiB and 4GiB, the kernel > > 896 MB to 4GB actually... 896MB to available phy. memory. > > This is what I have concluded > > 1. ZONE_NORMAL is from 16MB - 896MB. ==> address space usable by > > the kernel as I said address space usable by kernel ==> (3g to 4gb) > > Thats the physical memory, which is mapped from PAGE_OFFSET + 16MB. > > > 2. High memory is from 1GB-4GB ==> to access this part of memory > > kernel has to temporarily map pages from high memory into ZONE_NORMAL. > > So, ZONE_HIGHMEM must be from 1GB-4GB and not from 896 MiB – End > its 896 - end of ram....so if u have 4gb ram it will be 869-4gb :) > It is related in a way, if you have 2G/2G user/kernelspace split, then > you can have more physical memory in ZONE_NORMAL, i.e it will be 2GB - > 128MB if HIGHMEM enabled. This way the kernel is able to access more > physical memory directly (since ZONE_NORMAL is mapped directly), than > having to go through the overhead of kmap()/kmap_atomic(). > yes...but this reduces the size of the user space. -- Vivek Kutal -- To unsubscribe from this list: send an email with "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx Please read the FAQ at http://kernelnewbies.org/FAQ
