I'm interested in the difference between Highmem and Lowmem:
- Why is there such a differentiation?
- What do we gain by doing so?
- What features does each have?
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On a 32-bit architecture, the address space range for addressing RAM is:
0x00000000 - 0xffffffff
4'294'967'295 (4 GB).
The linux kernel splits that up 3/1 (could also be 2/2, or 1/3 1) into user space (high memory) and kernel space (low memory) respectively.
The user space range:
0x00000000 - 0xbfffffff
Every newly spawned user process gets an address (range) inside this area. User processes are generally untrusted and therefore are forbidden to access the kernel space. Further, they are considered non-urgent, as a general rule, the kernel tries to defer the allocation of memory to those processes.
The kernel space range:
0xc0000000 - 0xffffffff
A kernel processes gets its address (range) here. The kernel can directly access this 1 GB of addresses (well, not the full 1 GB, there are 128 MB reserved for high memory access).
Processes spawned in kernel space are trusted, urgent and assumed error-free, the memory request gets processed instantaneously.
Every kernel process can also access the user space range if it wishes to. And to achieve this, the kernel maps an address from the user space (the high memory) to its kernel space (the low memory), the 128 MB mentioned above are especially reserved for this.
1 Whether the split is 3/1, 2/2, or 1/3 is controlled by the
CONFIG_VMSPLIT_... option; you can probably check under
/boot/config* to see which option was selected for your kernel.
In an ideal world, every system component would be able to map all the memory it ever needs to access. And this is the case for processes on Linux and most operating systems: a 32-bit process can only access a little less than 2^32 bytes of virtual memory (in fact about 3GB on a typical Linux 32-bit architecture). It gets difficult for the kernel, which needs to be able to map the full memory of the process whose system call it's executing, plus the whole physical memory, plus any other memory-mapped hardware device.
So when a 32-bit kernel needs to map more than 4GB of memory, it must be compiled with high memory support. High memory is memory which is not permanently mapped in the kernel's address space. (Low memory is the opposite: it is always mapped, so you can access it in the kernel simply by dereferencing a pointer.)
When you access high memory from kernel code, you need to call
kmap first, to obtain a pointer from a page data structure (
struct page). Calling
kmap works whether the page is in high or low memory. There is also
kmap_atomic which has added constraints but is more efficient on multiprocessor machines because it uses finer-grained locking. The pointer obtained through
kmap is a resource: it uses up address space. Once you've finished with it, you must call
kunmap_atomic) to free that resource; then the pointer is no longer valid, and the contents of the page can't be accessed until you call
This is relevant to the Linux kernel; I'm not sure how any Unix kernel handles this.
The High Memory is the segment of memory that user-space programs can address. It cannot touch Low Memory.
Low Memory is the segment of memory that the Linux kernel can address directly. If the kernel must access High Memory, it has to map it into its own address space first.
There was a patch introduced recently that lets you control where the segment is. The tradeoff is that you can take addressable memory away from user space so that the kernel can have more memory that it does not have to map before using.
HIGHMEM is a range of kernel's memory space, but it is NOT memory you access but it's a place where you put what you want to access.
A typical 32bit Linux virtual memory map is like:
0x00000000-0xbfffffff: user process (3GB)
0xc0000000-0xffffffff: kernel space (1GB)
(CPU-specific vector and whatsoever are ignored here).
Linux splits the 1GB kernel space into 2 pieces, LOWMEM and HIGHMEM. The split varies from installation to installation.
If an installation chooses, say, 512MB-512MB for LOW and HIGH mems, the 512MB LOWMEM (0xc0000000-0xdfffffff) is statically mapped at the kernel boot time; usually the first so many bytes of the physical memory is used for this so that virtual and physical addresses in this range have a constant offset of, say, 0xc0000000.
On the other hand, the latter 512MB (HIGHMEM) has no static mapping (although you could leave pages semi-permanently mapped there, but you must do so explicitly in your driver code). Instead, pages are temporarily mapped and unmapped here so that virtual and physical addresses in this range have no consistent mapping. Typical uses of HIGHMEM include single-time data buffers.
For the people looking for an explanation in the context of Linux kernel memory space, beware that there are two conflicting definitions of the high/low memory split (unfortunately there is no standard, one has to interpret that in context):
"High memory" defined as the totality of kernel space in VIRTUAL memory. This is a region that only the kernel can access and comprises all virtual addresses greater or equal than
PAGE_OFFSET. "Low memory" refers therefore to the region of the remaining addresses, which correspond to the user-space memory accessible from each user process.
For example: on 32-bit x86 with a default
PAGE_OFFSET, this means that high memory is any address
ADDR ≥ 0xC0000000 = PAGE_OFFSET (i.e. higher 1 GB).
This is the reason why in Linux 32-bit processes are typically limited to 3 GB.
PAGE_OFFSET cannot be configured directly, it depends on the configurable
VMSPLIT_x options (source).
To summarize: in 32-bit archs, virtual memory is by default split into lower 3 GB (user space) and higher 1 GB (kernel space).
For 64 bit,
PAGE_OFFSET is not configurable and depends on architectural details that are sometimes detected at runtime during kernel load.
0xffff888000000000 for 4-level paging (typical) and
0xff11000000000000 for 5-level paging (source). For ARM64 this is usually
0x8000000000000000. Note though, if KASLR is enabled, this value is intentionally unpredictable.
"High memory" defined as the portion of PHYSICAL memory that cannot be mapped contiguously with the rest of the kernel virtual memory. A portion of the kernel virtual address space can be mapped as a single contiguous chunk into the so-called physical "low memory". To fully understand what this means, a deeper knowledge of the Linux virtual memory space is required. I would recommend going through these slides.
From the slides:
This kind of "high/low memory" split is only applicable to 32-bit architectures where the installed physical RAM size is relatively high (more than ~1 GB). Otherwise, i.e. when the physical address space is small (<1 GB) or when the virtual memory space is large (64 bits), the whole physical space can be accessed from the kernel virtual memory space. In that case, all physical memory is considered "low memory".
It is preferable that high memory does not exist at all because the whole physical space can be accessed directly from the kernel, which makes memory management a lot simpler and efficient. This is especially important when dealing with DMAs (which typically require physically contiguous memory).
See also the answer by @gilles
Many people have said that the low memory is for the operating system. This is usually true but does not have to be. High memory and low memory are just two parts of the memory space, but in Linux system low memory is only for kernel and high memory for user processes.
According to the "Dinosaur book (operating system concepts)", we can place the operating system in either low memory or high memory. The major factor affecting this decision is the location of the interrupt vector. Since the interrupt vector is often in low memory, programmers usually place the operating system in low memory as well.