highmem.rst (7197B)
1.. _highmem: 2 3==================== 4High Memory Handling 5==================== 6 7By: Peter Zijlstra <a.p.zijlstra@chello.nl> 8 9.. contents:: :local: 10 11What Is High Memory? 12==================== 13 14High memory (highmem) is used when the size of physical memory approaches or 15exceeds the maximum size of virtual memory. At that point it becomes 16impossible for the kernel to keep all of the available physical memory mapped 17at all times. This means the kernel needs to start using temporary mappings of 18the pieces of physical memory that it wants to access. 19 20The part of (physical) memory not covered by a permanent mapping is what we 21refer to as 'highmem'. There are various architecture dependent constraints on 22where exactly that border lies. 23 24In the i386 arch, for example, we choose to map the kernel into every process's 25VM space so that we don't have to pay the full TLB invalidation costs for 26kernel entry/exit. This means the available virtual memory space (4GiB on 27i386) has to be divided between user and kernel space. 28 29The traditional split for architectures using this approach is 3:1, 3GiB for 30userspace and the top 1GiB for kernel space:: 31 32 +--------+ 0xffffffff 33 | Kernel | 34 +--------+ 0xc0000000 35 | | 36 | User | 37 | | 38 +--------+ 0x00000000 39 40This means that the kernel can at most map 1GiB of physical memory at any one 41time, but because we need virtual address space for other things - including 42temporary maps to access the rest of the physical memory - the actual direct 43map will typically be less (usually around ~896MiB). 44 45Other architectures that have mm context tagged TLBs can have separate kernel 46and user maps. Some hardware (like some ARMs), however, have limited virtual 47space when they use mm context tags. 48 49 50Temporary Virtual Mappings 51========================== 52 53The kernel contains several ways of creating temporary mappings. The following 54list shows them in order of preference of use. 55 56* kmap_local_page(). This function is used to require short term mappings. 57 It can be invoked from any context (including interrupts) but the mappings 58 can only be used in the context which acquired them. 59 60 This function should be preferred, where feasible, over all the others. 61 62 These mappings are thread-local and CPU-local, meaning that the mapping 63 can only be accessed from within this thread and the thread is bound the 64 CPU while the mapping is active. Even if the thread is preempted (since 65 preemption is never disabled by the function) the CPU can not be 66 unplugged from the system via CPU-hotplug until the mapping is disposed. 67 68 It's valid to take pagefaults in a local kmap region, unless the context 69 in which the local mapping is acquired does not allow it for other reasons. 70 71 kmap_local_page() always returns a valid virtual address and it is assumed 72 that kunmap_local() will never fail. 73 74 Nesting kmap_local_page() and kmap_atomic() mappings is allowed to a certain 75 extent (up to KMAP_TYPE_NR) but their invocations have to be strictly ordered 76 because the map implementation is stack based. See kmap_local_page() kdocs 77 (included in the "Functions" section) for details on how to manage nested 78 mappings. 79 80* kmap_atomic(). This permits a very short duration mapping of a single 81 page. Since the mapping is restricted to the CPU that issued it, it 82 performs well, but the issuing task is therefore required to stay on that 83 CPU until it has finished, lest some other task displace its mappings. 84 85 kmap_atomic() may also be used by interrupt contexts, since it does not 86 sleep and the callers too may not sleep until after kunmap_atomic() is 87 called. 88 89 Each call of kmap_atomic() in the kernel creates a non-preemptible section 90 and disable pagefaults. This could be a source of unwanted latency. Therefore 91 users should prefer kmap_local_page() instead of kmap_atomic(). 92 93 It is assumed that k[un]map_atomic() won't fail. 94 95* kmap(). This should be used to make short duration mapping of a single 96 page with no restrictions on preemption or migration. It comes with an 97 overhead as mapping space is restricted and protected by a global lock 98 for synchronization. When mapping is no longer needed, the address that 99 the page was mapped to must be released with kunmap(). 100 101 Mapping changes must be propagated across all the CPUs. kmap() also 102 requires global TLB invalidation when the kmap's pool wraps and it might 103 block when the mapping space is fully utilized until a slot becomes 104 available. Therefore, kmap() is only callable from preemptible context. 105 106 All the above work is necessary if a mapping must last for a relatively 107 long time but the bulk of high-memory mappings in the kernel are 108 short-lived and only used in one place. This means that the cost of 109 kmap() is mostly wasted in such cases. kmap() was not intended for long 110 term mappings but it has morphed in that direction and its use is 111 strongly discouraged in newer code and the set of the preceding functions 112 should be preferred. 113 114 On 64-bit systems, calls to kmap_local_page(), kmap_atomic() and kmap() have 115 no real work to do because a 64-bit address space is more than sufficient to 116 address all the physical memory whose pages are permanently mapped. 117 118* vmap(). This can be used to make a long duration mapping of multiple 119 physical pages into a contiguous virtual space. It needs global 120 synchronization to unmap. 121 122 123Cost of Temporary Mappings 124========================== 125 126The cost of creating temporary mappings can be quite high. The arch has to 127manipulate the kernel's page tables, the data TLB and/or the MMU's registers. 128 129If CONFIG_HIGHMEM is not set, then the kernel will try and create a mapping 130simply with a bit of arithmetic that will convert the page struct address into 131a pointer to the page contents rather than juggling mappings about. In such a 132case, the unmap operation may be a null operation. 133 134If CONFIG_MMU is not set, then there can be no temporary mappings and no 135highmem. In such a case, the arithmetic approach will also be used. 136 137 138i386 PAE 139======== 140 141The i386 arch, under some circumstances, will permit you to stick up to 64GiB 142of RAM into your 32-bit machine. This has a number of consequences: 143 144* Linux needs a page-frame structure for each page in the system and the 145 pageframes need to live in the permanent mapping, which means: 146 147* you can have 896M/sizeof(struct page) page-frames at most; with struct 148 page being 32-bytes that would end up being something in the order of 112G 149 worth of pages; the kernel, however, needs to store more than just 150 page-frames in that memory... 151 152* PAE makes your page tables larger - which slows the system down as more 153 data has to be accessed to traverse in TLB fills and the like. One 154 advantage is that PAE has more PTE bits and can provide advanced features 155 like NX and PAT. 156 157The general recommendation is that you don't use more than 8GiB on a 32-bit 158machine - although more might work for you and your workload, you're pretty 159much on your own - don't expect kernel developers to really care much if things 160come apart. 161 162 163Functions 164========= 165 166.. kernel-doc:: include/linux/highmem.h 167.. kernel-doc:: include/linux/highmem-internal.h