If you see the core centos high meme, this guide should help you.
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In 32-bit architecture, the address space for addressing RAM:
0x00000000 - 0xffffffff
The Linux kernel divides exactly 3/1 (can also be 2/2 or 1/3 1 ) to provide user space (high memory) or kernel space (less memory).
0x00000000 - New 0xbffffffff
every generated user process gets a valid (zone) in that zone. Users are processes that typically They are not trustworthy and therefore do not have access to kernel space. Moreover, they are not considered urgent. Typically, the kernel often tries to defer the allocation of the memory most frequently allocated to these processes.
0xc0000000 0xffffffff
A – Kernel actions get their address (zone) here. The kernel can directly access this 1GB address contact information (well, not the whole 1GB, 128MB is reserved for large memory access).
Processes that are spawned in kernel space are actually trustworthy, urgent and considered error-free, the required memory is processed immediately.
Each kernel process can also access a user space region as follows. And to achieve that capacity, the kernel maps the address from all user space (high memory) to its own kernel space (low memory), our 128MB above is especially suited for this.
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1 Whether the share is 3/1, 2/2, or 1/3, the CONFIG_VMSPLIT _... option is used; You are probably You can check / boot / config * which option was selected for your kernel.
The first thing to look at is the Linux device drivers (available both online and in confirmation form), in particular Chapter 15, which has a section on this topic.
In an ideal world, every component in the system could map all the memory needed to access it. And this is only in the case of processes running Linux and most operating systems: a 32-bit process can only access 2 ^ 32 bytes of allocated memory (actually about 3 GB on a stunning arch(see a typical 32-bit Linux architecture). This becomes very difficult for the kernel, which usually has to be able to allocate all the memory of the called process, as well as all the physical memory and any other memory-mapped device hardware.
So, if the latest 32-bit kernel needs to map more than 4GB of memory, it needs to be compiled with high memory support. High memory is memory that is not permanently mapped to the kernel address space. (Lack of memory is just the opposite: it is always displayed, so you can only access it in the kernel by dereferencing the pointer.)
If you are accessing high memory through kernel code, you must first call kmap to get a hint from the data page structure ( struct page ). The kmap call works regardless of whether the page has a lot of memory or not. Of course, there is kmap_atomic , which has additional restrictions, but is more efficient on multiprocessor machines simply because it uses granular locking. The pointer passed to kmap is a resource: it has an address space. Once you’re done, you need to call kunmap kunmap_atomic ) (or deallocate that resource directly; then the suggestion is no longer valid and the information on the page cannot be called if you again don’t call kmap .
answered Dec 17, 2010 at 20:55
This is related to the Linux kernel; I don’t know how the Unix kernel handles this. High
Memory is a segment of memory that can be accessed by user-space programs. He cannot touch too little Memory. Memory
low is a segment of memory that the Linux kernel can talk about directly. When the kernel needs to access high memory, it must first map it to your dog’s address space.
A feature fix was recently introduced to allow you to specify where the segment is located. Often the tradeoff is thatyou can remove addressable memory from user space so that our own kernel has more memory than it actually needs to allocate before use.
http://tldp.org/HOWTO/KernelAnalysis-HOWTO-7.html
http://linux-mm.org/HighMemory
answered Dec 17 ’10 at 12:50
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HIGHMEM is an area outside of the kernel memory area, but this is NOT the memory you are accessing, but the place where you put whatever you want to recover.
0x00000000-0xbfffffff: User Process (3GB)
0xc0000000-0xffffffff: kernel memory (1 GB)
Linux divides 1 GB of kernel memory into 2 parts: LOWMEM and HIGHMEM. The separation varies depending on theFrom installation. When installing.
If the installation chooses 512MB-512MB for LOW and HIGH memes, declare a new 512MB LOWMEM (0xc0000000-0xdffffffff) statically allocated at kernel boot time; Typically, this is done using a base of so many bytes of physical RAM that the allocated and physical addresses in this area provide a constant offset, for example, 0xc0000000.
On the other hand, this last 512MB (HIGHMEM) does not provide any static mapping (although you can display semi-persistently mapped pages to it, but you must do so explicitly in your biker code). Instead, pages are temporarily assigned and not assigned here, so the virtual and physical descriptors in this area are not permanently assigned. Typical uses for HIGHMEM include one-time result buffers.
