Based on kernel version 4.15. Page generated on 2018-01-29 10:00 EST.
1 ============================= 2 No-MMU memory mapping support 3 ============================= 4 5 The kernel has limited support for memory mapping under no-MMU conditions, such 6 as are used in uClinux environments. From the userspace point of view, memory 7 mapping is made use of in conjunction with the mmap() system call, the shmat() 8 call and the execve() system call. From the kernel's point of view, execve() 9 mapping is actually performed by the binfmt drivers, which call back into the 10 mmap() routines to do the actual work. 11 12 Memory mapping behaviour also involves the way fork(), vfork(), clone() and 13 ptrace() work. Under uClinux there is no fork(), and clone() must be supplied 14 the CLONE_VM flag. 15 16 The behaviour is similar between the MMU and no-MMU cases, but not identical; 17 and it's also much more restricted in the latter case: 18 19 (#) Anonymous mapping, MAP_PRIVATE 20 21 In the MMU case: VM regions backed by arbitrary pages; copy-on-write 22 across fork. 23 24 In the no-MMU case: VM regions backed by arbitrary contiguous runs of 25 pages. 26 27 (#) Anonymous mapping, MAP_SHARED 28 29 These behave very much like private mappings, except that they're 30 shared across fork() or clone() without CLONE_VM in the MMU case. Since 31 the no-MMU case doesn't support these, behaviour is identical to 32 MAP_PRIVATE there. 33 34 (#) File, MAP_PRIVATE, PROT_READ / PROT_EXEC, !PROT_WRITE 35 36 In the MMU case: VM regions backed by pages read from file; changes to 37 the underlying file are reflected in the mapping; copied across fork. 38 39 In the no-MMU case: 40 41 - If one exists, the kernel will re-use an existing mapping to the 42 same segment of the same file if that has compatible permissions, 43 even if this was created by another process. 44 45 - If possible, the file mapping will be directly on the backing device 46 if the backing device has the NOMMU_MAP_DIRECT capability and 47 appropriate mapping protection capabilities. Ramfs, romfs, cramfs 48 and mtd might all permit this. 49 50 - If the backing device device can't or won't permit direct sharing, 51 but does have the NOMMU_MAP_COPY capability, then a copy of the 52 appropriate bit of the file will be read into a contiguous bit of 53 memory and any extraneous space beyond the EOF will be cleared 54 55 - Writes to the file do not affect the mapping; writes to the mapping 56 are visible in other processes (no MMU protection), but should not 57 happen. 58 59 (#) File, MAP_PRIVATE, PROT_READ / PROT_EXEC, PROT_WRITE 60 61 In the MMU case: like the non-PROT_WRITE case, except that the pages in 62 question get copied before the write actually happens. From that point 63 on writes to the file underneath that page no longer get reflected into 64 the mapping's backing pages. The page is then backed by swap instead. 65 66 In the no-MMU case: works much like the non-PROT_WRITE case, except 67 that a copy is always taken and never shared. 68 69 (#) Regular file / blockdev, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE 70 71 In the MMU case: VM regions backed by pages read from file; changes to 72 pages written back to file; writes to file reflected into pages backing 73 mapping; shared across fork. 74 75 In the no-MMU case: not supported. 76 77 (#) Memory backed regular file, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE 78 79 In the MMU case: As for ordinary regular files. 80 81 In the no-MMU case: The filesystem providing the memory-backed file 82 (such as ramfs or tmpfs) may choose to honour an open, truncate, mmap 83 sequence by providing a contiguous sequence of pages to map. In that 84 case, a shared-writable memory mapping will be possible. It will work 85 as for the MMU case. If the filesystem does not provide any such 86 support, then the mapping request will be denied. 87 88 (#) Memory backed blockdev, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE 89 90 In the MMU case: As for ordinary regular files. 91 92 In the no-MMU case: As for memory backed regular files, but the 93 blockdev must be able to provide a contiguous run of pages without 94 truncate being called. The ramdisk driver could do this if it allocated 95 all its memory as a contiguous array upfront. 96 97 (#) Memory backed chardev, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE 98 99 In the MMU case: As for ordinary regular files. 100 101 In the no-MMU case: The character device driver may choose to honour 102 the mmap() by providing direct access to the underlying device if it 103 provides memory or quasi-memory that can be accessed directly. Examples 104 of such are frame buffers and flash devices. If the driver does not 105 provide any such support, then the mapping request will be denied. 106 107 108 Further notes on no-MMU MMAP 109 ============================ 110 111 (#) A request for a private mapping of a file may return a buffer that is not 112 page-aligned. This is because XIP may take place, and the data may not be 113 paged aligned in the backing store. 114 115 (#) A request for an anonymous mapping will always be page aligned. If 116 possible the size of the request should be a power of two otherwise some 117 of the space may be wasted as the kernel must allocate a power-of-2 118 granule but will only discard the excess if appropriately configured as 119 this has an effect on fragmentation. 120 121 (#) The memory allocated by a request for an anonymous mapping will normally 122 be cleared by the kernel before being returned in accordance with the 123 Linux man pages (ver 2.22 or later). 124 125 In the MMU case this can be achieved with reasonable performance as 126 regions are backed by virtual pages, with the contents only being mapped 127 to cleared physical pages when a write happens on that specific page 128 (prior to which, the pages are effectively mapped to the global zero page 129 from which reads can take place). This spreads out the time it takes to 130 initialize the contents of a page - depending on the write-usage of the 131 mapping. 132 133 In the no-MMU case, however, anonymous mappings are backed by physical 134 pages, and the entire map is cleared at allocation time. This can cause 135 significant delays during a userspace malloc() as the C library does an 136 anonymous mapping and the kernel then does a memset for the entire map. 137 138 However, for memory that isn't required to be precleared - such as that 139 returned by malloc() - mmap() can take a MAP_UNINITIALIZED flag to 140 indicate to the kernel that it shouldn't bother clearing the memory before 141 returning it. Note that CONFIG_MMAP_ALLOW_UNINITIALIZED must be enabled 142 to permit this, otherwise the flag will be ignored. 143 144 uClibc uses this to speed up malloc(), and the ELF-FDPIC binfmt uses this 145 to allocate the brk and stack region. 146 147 (#) A list of all the private copy and anonymous mappings on the system is 148 visible through /proc/maps in no-MMU mode. 149 150 (#) A list of all the mappings in use by a process is visible through 151 /proc/<pid>/maps in no-MMU mode. 152 153 (#) Supplying MAP_FIXED or a requesting a particular mapping address will 154 result in an error. 155 156 (#) Files mapped privately usually have to have a read method provided by the 157 driver or filesystem so that the contents can be read into the memory 158 allocated if mmap() chooses not to map the backing device directly. An 159 error will result if they don't. This is most likely to be encountered 160 with character device files, pipes, fifos and sockets. 161 162 163 Interprocess shared memory 164 ========================== 165 166 Both SYSV IPC SHM shared memory and POSIX shared memory is supported in NOMMU 167 mode. The former through the usual mechanism, the latter through files created 168 on ramfs or tmpfs mounts. 169 170 171 Futexes 172 ======= 173 174 Futexes are supported in NOMMU mode if the arch supports them. An error will 175 be given if an address passed to the futex system call lies outside the 176 mappings made by a process or if the mapping in which the address lies does not 177 support futexes (such as an I/O chardev mapping). 178 179 180 No-MMU mremap 181 ============= 182 183 The mremap() function is partially supported. It may change the size of a 184 mapping, and may move it [#]_ if MREMAP_MAYMOVE is specified and if the new size 185 of the mapping exceeds the size of the slab object currently occupied by the 186 memory to which the mapping refers, or if a smaller slab object could be used. 187 188 MREMAP_FIXED is not supported, though it is ignored if there's no change of 189 address and the object does not need to be moved. 190 191 Shared mappings may not be moved. Shareable mappings may not be moved either, 192 even if they are not currently shared. 193 194 The mremap() function must be given an exact match for base address and size of 195 a previously mapped object. It may not be used to create holes in existing 196 mappings, move parts of existing mappings or resize parts of mappings. It must 197 act on a complete mapping. 198 199 .. [#] Not currently supported. 200 201 202 Providing shareable character device support 203 ============================================ 204 205 To provide shareable character device support, a driver must provide a 206 file->f_op->get_unmapped_area() operation. The mmap() routines will call this 207 to get a proposed address for the mapping. This may return an error if it 208 doesn't wish to honour the mapping because it's too long, at a weird offset, 209 under some unsupported combination of flags or whatever. 210 211 The driver should also provide backing device information with capabilities set 212 to indicate the permitted types of mapping on such devices. The default is 213 assumed to be readable and writable, not executable, and only shareable 214 directly (can't be copied). 215 216 The file->f_op->mmap() operation will be called to actually inaugurate the 217 mapping. It can be rejected at that point. Returning the ENOSYS error will 218 cause the mapping to be copied instead if NOMMU_MAP_COPY is specified. 219 220 The vm_ops->close() routine will be invoked when the last mapping on a chardev 221 is removed. An existing mapping will be shared, partially or not, if possible 222 without notifying the driver. 223 224 It is permitted also for the file->f_op->get_unmapped_area() operation to 225 return -ENOSYS. This will be taken to mean that this operation just doesn't 226 want to handle it, despite the fact it's got an operation. For instance, it 227 might try directing the call to a secondary driver which turns out not to 228 implement it. Such is the case for the framebuffer driver which attempts to 229 direct the call to the device-specific driver. Under such circumstances, the 230 mapping request will be rejected if NOMMU_MAP_COPY is not specified, and a 231 copy mapped otherwise. 232 233 .. important:: 234 235 Some types of device may present a different appearance to anyone 236 looking at them in certain modes. Flash chips can be like this; for 237 instance if they're in programming or erase mode, you might see the 238 status reflected in the mapping, instead of the data. 239 240 In such a case, care must be taken lest userspace see a shared or a 241 private mapping showing such information when the driver is busy 242 controlling the device. Remember especially: private executable 243 mappings may still be mapped directly off the device under some 244 circumstances! 245 246 247 Providing shareable memory-backed file support 248 ============================================== 249 250 Provision of shared mappings on memory backed files is similar to the provision 251 of support for shared mapped character devices. The main difference is that the 252 filesystem providing the service will probably allocate a contiguous collection 253 of pages and permit mappings to be made on that. 254 255 It is recommended that a truncate operation applied to such a file that 256 increases the file size, if that file is empty, be taken as a request to gather 257 enough pages to honour a mapping. This is required to support POSIX shared 258 memory. 259 260 Memory backed devices are indicated by the mapping's backing device info having 261 the memory_backed flag set. 262 263 264 Providing shareable block device support 265 ======================================== 266 267 Provision of shared mappings on block device files is exactly the same as for 268 character devices. If there isn't a real device underneath, then the driver 269 should allocate sufficient contiguous memory to honour any supported mapping. 270 271 272 Adjusting page trimming behaviour 273 ================================= 274 275 NOMMU mmap automatically rounds up to the nearest power-of-2 number of pages 276 when performing an allocation. This can have adverse effects on memory 277 fragmentation, and as such, is left configurable. The default behaviour is to 278 aggressively trim allocations and discard any excess pages back in to the page 279 allocator. In order to retain finer-grained control over fragmentation, this 280 behaviour can either be disabled completely, or bumped up to a higher page 281 watermark where trimming begins. 282 283 Page trimming behaviour is configurable via the sysctl ``vm.nr_trim_pages``.