Originally, it is a part of the Linux Kernel source code, so it will be treated as GPLv2 (recognition that it should be).
https://www.kernel.org/doc/html/latest/index.html
Licensing documentation
The following describes the license of the Linux kernel source code (GPLv2), how to properly mark the license of individual files in the source tree, as well as links to the full license text.
https://www.kernel.org/doc/html/latest/process/license-rules.html#kernel-licensing
https://www.kernel.org/doc/html/latest/vm/highmem.html
High Memory Handling By: Peter Zijlstra [email protected]
What Is High Memory?
High memory (highmem) is used when the size of physical memory approaches or exceeds the maximum size of virtual memory.
High memory (highmem) is used when the size of physical memory approaches or exceeds the maximum size of virtual memory.
At that point it becomes impossible for the kernel to keep all of the available physical memory mapped at all times.
At that point, the kernel will no longer be able to keep all available physical memory mapped at all times.
This means the kernel needs to start using temporary mappings of the pieces of physical memory that it wants to access.
This means that you need to temporarily start mapping some of the physical memory that the kernel wants to access.
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The part of (physical) memory not covered by a permanent mapping is what we refer to as ‘highmem’.
The portion of (physical) memory that is not covered by persistent mapping alone is called highmem. ..
There are various architecture dependent constraints on where exactly that border lies.
There are various architecture-dependent constraints on where the boundaries are located.
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In the i386 arch, for example, we choose to map the kernel into every process’s VM space so that we don’t have to pay the full TLB invalidation costs for kernel entry/exit.
For example, for the i386 arche, choose to map the kernel to the virtual machine space for all processes. We no longer have to pay the full TLB invalidation cost for kernel entry / exit.
This means the available virtual memory space (4GiB on i386) has to be divided between user and kernel space.
This means that the available virtual memory space (4GiB on i386) is split between the user and kernel space.
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The traditional split for architectures using this approach is 3:1, 3GiB for userspace and the top 1GiB for kernel space:
The traditional split method for architectures that take this approach is 3: 1 with 3GiB in user space and top 1GiB in kernel space.
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+--------+ 0xffffffff
| Kernel |
+--------+ 0xc0000000
| |
| User |
| |
+--------+ 0x00000000
This means that the kernel can at most map 1GiB of physical memory at any one time, but because we need virtual address space for other things - including temporary maps to access the rest of the physical memory - the actual direct map will typically be less (usually around ~896MiB).
This means that the kernel can map up to 1 GiB of physical memory at a time. The actual direct map is usually less than that (usually around 896MiB) because it needs virtual address space for other things, such as a temporary map to access the rest of the physical memory.
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Other architectures that have mm context tagged TLBs can have separate kernel and user maps. Some hardware (like some ARMs), however, have limited virtual space when they use mm context tags.
Other architectures with mm context tagged TLBs can have a split kernel and user map. However, on some hardware (for example, part of ARM), using mm context tags will limit the virtual space.
Temporary Virtual Mappings
The kernel contains several ways of creating temporary mappings:
The kernel has several ways to generate temporary mappings.
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- vmap(). This can be used to make a long duration mapping of multiple physical pages into a contiguous virtual space. It needs global synchronization to unmap.
- kmap(). This permits a short duration mapping of a single page. It needs global synchronization, but is amortized somewhat. It is also prone to deadlocks when using in a nested fashion, and so it is not recommended for new code.
- kmap_atomic(). This permits a very short duration mapping of a single page. Since the mapping is restricted to the CPU that issued it, it performs well, but the issuing task is therefore required to stay on that CPU until it has finished, lest some other task displace its mappings. . kmap_atomic() may also be used by interrupt contexts, since it is does not sleep and the caller may not sleep until after kunmap_atomic() is called. . It may be assumed that k[un]map_atomic() won’t fail.
Using kmap_atomic
When and where to use kmap_atomic() is straightforward.
It is clear when and where to use kmap_atomic ().
It is used when code wants to access the contents of a page that might be allocated from high memory (see __GFP_HIGHMEM), for example a page in the pagecache.
This is used when the code accesses pages reserved by high memory (__ GFP_HIGHMEM), such as pages in the page cache.
The API has two functions, and they can be used in a manner similar to the following
The API has two functions, each of which is used as follows.
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/* Find the page of interest. */
struct page *page = find_get_page(mapping, offset);
/* Gain access to the contents of that page. */
void *vaddr = kmap_atomic(page);
/* Do something to the contents of that page. */
memset(vaddr, 0, PAGE_SIZE);
/* Unmap that page. */
kunmap_atomic(vaddr);
Note that the kunmap_atomic() call takes the result of the kmap_atomic() call not the argument.
Be careful that kummap_atomic () is called regardless of the result of calling kmap_atomic ().
If you need to map two pages because you want to copy from one page to another you need to keep the kmap_atomic calls strictly nested, like:
If you want to map two pages and copy from one page to another, you need to call kmap_atomic () with a strict nesting structure. For example
vaddr1 = kmap_atomic(page1);
vaddr2 = kmap_atomic(page2);
memcpy(vaddr1, vaddr2, PAGE_SIZE);
kunmap_atomic(vaddr2);
kunmap_atomic(vaddr1);
Cost of Temporary Mappings
The cost of creating temporary mappings can be quite high.
The cost of generating a temporary mapping can be very high.
The arch has to manipulate the kernel’s page tables, the data TLB and/or the MMU’s registers.
The architecture requires working with the kernel page table, data TLB and / and the MMU register.
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If CONFIG_HIGHMEM is not set, then the kernel will try and create a mapping simply with a bit of arithmetic that will convert the page struct address into a pointer to the page contents rather than juggling mappings about. In such a case, the unmap operation may be a null operation.
If CONFIG_HIGHMEM is not set, the kernel will try to create a mapping with a few arithmetic operations that translate the address of the page structure into a pointer to the contents of the page instead of juggling the mapping. In such cases, the unmap operation can be a null operation.
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If CONFIG_MMU is not set, then there can be no temporary mappings and no highmem. In such a case, the arithmetic approach will also be used.
If CONFIG_MMU is not set, there may be no temporary mapping or highmem. Even in such cases, the arithmetic approach is used.
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i386 PAE
The i386 arch, under some circumstances, will permit you to stick up to 64GiB of RAM into your 32-bit machine. This has a number of consequences:
Depending on the situation, the i386 Architect Tea can also install up to 64 GiB of RAM on a 32-bit machine. This has many implications.
- Linux needs a page-frame structure for each page in the system and the pageframes need to live in the permanent mapping, which means:
- you can have 896M/sizeof(struct page) page-frames at most; with struct page being 32-bytes that would end up being something in the order of 112G worth of pages; the kernel, however, needs to store more than just page-frames in that memory…
- PAE makes your page tables larger - which slows the system down as more data has to be accessed to traverse in TLB fills and the like. One advantage is that PAE has more PTE bits and can provide advanced features like NX and PAT.
The general recommendation is that you don’t use more than 8GiB on a 32-bit machine - although more might work for you and your workload, you’re pretty much on your own - don’t expect kernel developers to really care much if things come apart.
A general recommendation is not to use more than 8GiB on a 32-bit machine. This may have an effect on users and user load. But do it yourself. If something goes wrong, don't really expect kernel developers to care.