1==================== 2High Memory Handling 3==================== 4 5By: Peter Zijlstra <a.p.zijlstra@chello.nl> 6 7.. contents:: :local: 8 9What Is High Memory? 10==================== 11 12High memory (highmem) is used when the size of physical memory approaches or 13exceeds the maximum size of virtual memory. At that point it becomes 14impossible for the kernel to keep all of the available physical memory mapped 15at all times. This means the kernel needs to start using temporary mappings of 16the pieces of physical memory that it wants to access. 17 18The part of (physical) memory not covered by a permanent mapping is what we 19refer to as 'highmem'. There are various architecture dependent constraints on 20where exactly that border lies. 21 22In the i386 arch, for example, we choose to map the kernel into every process's 23VM space so that we don't have to pay the full TLB invalidation costs for 24kernel entry/exit. This means the available virtual memory space (4GiB on 25i386) has to be divided between user and kernel space. 26 27The traditional split for architectures using this approach is 3:1, 3GiB for 28userspace and the top 1GiB for kernel space:: 29 30 +--------+ 0xffffffff 31 | Kernel | 32 +--------+ 0xc0000000 33 | | 34 | User | 35 | | 36 +--------+ 0x00000000 37 38This means that the kernel can at most map 1GiB of physical memory at any one 39time, but because we need virtual address space for other things - including 40temporary maps to access the rest of the physical memory - the actual direct 41map will typically be less (usually around ~896MiB). 42 43Other architectures that have mm context tagged TLBs can have separate kernel 44and user maps. Some hardware (like some ARMs), however, have limited virtual 45space when they use mm context tags. 46 47 48Temporary Virtual Mappings 49========================== 50 51The kernel contains several ways of creating temporary mappings. The following 52list shows them in order of preference of use. 53 54* kmap_local_page(). This function is used to require short term mappings. 55 It can be invoked from any context (including interrupts) but the mappings 56 can only be used in the context which acquired them. 57 58 This function should always be used, whereas kmap_atomic() and kmap() have 59 been deprecated. 60 61 These mappings are thread-local and CPU-local, meaning that the mapping 62 can only be accessed from within this thread and the thread is bound to the 63 CPU while the mapping is active. Although preemption is never disabled by 64 this function, the CPU can not be unplugged from the system via 65 CPU-hotplug until the mapping is disposed. 66 67 It's valid to take pagefaults in a local kmap region, unless the context 68 in which the local mapping is acquired does not allow it for other reasons. 69 70 As said, pagefaults and preemption are never disabled. There is no need to 71 disable preemption because, when context switches to a different task, the 72 maps of the outgoing task are saved and those of the incoming one are 73 restored. 74 75 kmap_local_page() always returns a valid virtual address and it is assumed 76 that kunmap_local() will never fail. 77 78 On CONFIG_HIGHMEM=n kernels and for low memory pages this returns the 79 virtual address of the direct mapping. Only real highmem pages are 80 temporarily mapped. Therefore, users may call a plain page_address() 81 for pages which are known to not come from ZONE_HIGHMEM. However, it is 82 always safe to use kmap_local_page() / kunmap_local(). 83 84 While it is significantly faster than kmap(), for the highmem case it 85 comes with restrictions about the pointers validity. Contrary to kmap() 86 mappings, the local mappings are only valid in the context of the caller 87 and cannot be handed to other contexts. This implies that users must 88 be absolutely sure to keep the use of the return address local to the 89 thread which mapped it. 90 91 Most code can be designed to use thread local mappings. User should 92 therefore try to design their code to avoid the use of kmap() by mapping 93 pages in the same thread the address will be used and prefer 94 kmap_local_page(). 95 96 Nesting kmap_local_page() and kmap_atomic() mappings is allowed to a certain 97 extent (up to KMAP_TYPE_NR) but their invocations have to be strictly ordered 98 because the map implementation is stack based. See kmap_local_page() kdocs 99 (included in the "Functions" section) for details on how to manage nested 100 mappings. 101 102* kmap_atomic(). This function has been deprecated; use kmap_local_page(). 103 104 NOTE: Conversions to kmap_local_page() must take care to follow the mapping 105 restrictions imposed on kmap_local_page(). Furthermore, the code between 106 calls to kmap_atomic() and kunmap_atomic() may implicitly depend on the side 107 effects of atomic mappings, i.e. disabling page faults or preemption, or both. 108 In that case, explicit calls to pagefault_disable() or preempt_disable() or 109 both must be made in conjunction with the use of kmap_local_page(). 110 111 [Legacy documentation] 112 113 This permits a very short duration mapping of a single page. Since the 114 mapping is restricted to the CPU that issued it, it performs well, but 115 the issuing task is therefore required to stay on that CPU until it has 116 finished, lest some other task displace its mappings. 117 118 kmap_atomic() may also be used by interrupt contexts, since it does not 119 sleep and the callers too may not sleep until after kunmap_atomic() is 120 called. 121 122 Each call of kmap_atomic() in the kernel creates a non-preemptible section 123 and disable pagefaults. This could be a source of unwanted latency. Therefore 124 users should prefer kmap_local_page() instead of kmap_atomic(). 125 126 It is assumed that k[un]map_atomic() won't fail. 127 128* kmap(). This function has been deprecated; use kmap_local_page(). 129 130 NOTE: Conversions to kmap_local_page() must take care to follow the mapping 131 restrictions imposed on kmap_local_page(). In particular, it is necessary to 132 make sure that the kernel virtual memory pointer is only valid in the thread 133 that obtained it. 134 135 [Legacy documentation] 136 137 This should be used to make short duration mapping of a single page with no 138 restrictions on preemption or migration. It comes with an overhead as mapping 139 space is restricted and protected by a global lock for synchronization. When 140 mapping is no longer needed, the address that the page was mapped to must be 141 released with kunmap(). 142 143 Mapping changes must be propagated across all the CPUs. kmap() also 144 requires global TLB invalidation when the kmap's pool wraps and it might 145 block when the mapping space is fully utilized until a slot becomes 146 available. Therefore, kmap() is only callable from preemptible context. 147 148 All the above work is necessary if a mapping must last for a relatively 149 long time but the bulk of high-memory mappings in the kernel are 150 short-lived and only used in one place. This means that the cost of 151 kmap() is mostly wasted in such cases. kmap() was not intended for long 152 term mappings but it has morphed in that direction and its use is 153 strongly discouraged in newer code and the set of the preceding functions 154 should be preferred. 155 156 On 64-bit systems, calls to kmap_local_page(), kmap_atomic() and kmap() have 157 no real work to do because a 64-bit address space is more than sufficient to 158 address all the physical memory whose pages are permanently mapped. 159 160* vmap(). This can be used to make a long duration mapping of multiple 161 physical pages into a contiguous virtual space. It needs global 162 synchronization to unmap. 163 164 165Cost of Temporary Mappings 166========================== 167 168The cost of creating temporary mappings can be quite high. The arch has to 169manipulate the kernel's page tables, the data TLB and/or the MMU's registers. 170 171If CONFIG_HIGHMEM is not set, then the kernel will try and create a mapping 172simply with a bit of arithmetic that will convert the page struct address into 173a pointer to the page contents rather than juggling mappings about. In such a 174case, the unmap operation may be a null operation. 175 176If CONFIG_MMU is not set, then there can be no temporary mappings and no 177highmem. In such a case, the arithmetic approach will also be used. 178 179 180i386 PAE 181======== 182 183The i386 arch, under some circumstances, will permit you to stick up to 64GiB 184of RAM into your 32-bit machine. This has a number of consequences: 185 186* Linux needs a page-frame structure for each page in the system and the 187 pageframes need to live in the permanent mapping, which means: 188 189* you can have 896M/sizeof(struct page) page-frames at most; with struct 190 page being 32-bytes that would end up being something in the order of 112G 191 worth of pages; the kernel, however, needs to store more than just 192 page-frames in that memory... 193 194* PAE makes your page tables larger - which slows the system down as more 195 data has to be accessed to traverse in TLB fills and the like. One 196 advantage is that PAE has more PTE bits and can provide advanced features 197 like NX and PAT. 198 199The general recommendation is that you don't use more than 8GiB on a 32-bit 200machine - although more might work for you and your workload, you're pretty 201much on your own - don't expect kernel developers to really care much if things 202come apart. 203 204 205Functions 206========= 207 208.. kernel-doc:: include/linux/highmem.h 209.. kernel-doc:: include/linux/highmem-internal.h 210