Documentation/nommu-mmap.txt

   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 BDI_CAP_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 BDI_CAP_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 ============================
 109 FURTHER NOTES ON NO-MMU MMAP
 110 ============================
 111
 112  (*) A request for a private mapping of a file may return a buffer that is not
 113      page-aligned.  This is because XIP may take place, and the data may not be
 114      paged aligned in the backing store.
 115
 116  (*) A request for an anonymous mapping will always be page aligned.  If
 117      possible the size of the request should be a power of two otherwise some
 118      of the space may be wasted as the kernel must allocate a power-of-2
 119      granule but will only discard the excess if appropriately configured as
 120      this has an effect on fragmentation.
 121
 122  (*) The memory allocated by a request for an anonymous mapping will normally
 123      be cleared by the kernel before being returned in accordance with the
 124      Linux man pages (ver 2.22 or later).
 125
 126      In the MMU case this can be achieved with reasonable performance as
 127      regions are backed by virtual pages, with the contents only being mapped
 128      to cleared physical pages when a write happens on that specific page
 129      (prior to which, the pages are effectively mapped to the global zero page
 130      from which reads can take place).  This spreads out the time it takes to
 131      initialize the contents of a page - depending on the write-usage of the
 132      mapping.
 133
 134      In the no-MMU case, however, anonymous mappings are backed by physical
 135      pages, and the entire map is cleared at allocation time.  This can cause
 136      significant delays during a userspace malloc() as the C library does an
 137      anonymous mapping and the kernel then does a memset for the entire map.
 138
 139      However, for memory that isn't required to be precleared - such as that
 140      returned by malloc() - mmap() can take a MAP_UNINITIALIZED flag to
 141      indicate to the kernel that it shouldn't bother clearing the memory before
 142      returning it.  Note that CONFIG_MMAP_ALLOW_UNINITIALIZED must be enabled
 143      to permit this, otherwise the flag will be ignored.
 144
 145      uClibc uses this to speed up malloc(), and the ELF-FDPIC binfmt uses this
 146      to allocate the brk and stack region.
 147
 148  (*) A list of all the private copy and anonymous mappings on the system is
 149      visible through /proc/maps in no-MMU mode.
 150
 151  (*) A list of all the mappings in use by a process is visible through
 152      /proc/<pid>/maps in no-MMU mode.
 153
 154  (*) Supplying MAP_FIXED or a requesting a particular mapping address will
 155      result in an error.
 156
 157  (*) Files mapped privately usually have to have a read method provided by the
 158      driver or filesystem so that the contents can be read into the memory
 159      allocated if mmap() chooses not to map the backing device directly. An
 160      error will result if they don't. This is most likely to be encountered
 161      with character device files, pipes, fifos and sockets.
 162
 163
 164 ==========================
 165 INTERPROCESS SHARED MEMORY
 166 ==========================
 167
 168 Both SYSV IPC SHM shared memory and POSIX shared memory is supported in NOMMU
 169 mode.  The former through the usual mechanism, the latter through files created
 170 on ramfs or tmpfs mounts.
 171
 172
 173 =======
 174 FUTEXES
 175 =======
 176
 177 Futexes are supported in NOMMU mode if the arch supports them.  An error will
 178 be given if an address passed to the futex system call lies outside the
 179 mappings made by a process or if the mapping in which the address lies does not
 180 support futexes (such as an I/O chardev mapping).
 181
 182
 183 =============
 184 NO-MMU MREMAP
 185 =============
 186
 187 The mremap() function is partially supported.  It may change the size of a
 188 mapping, and may move it[*] if MREMAP_MAYMOVE is specified and if the new size
 189 of the mapping exceeds the size of the slab object currently occupied by the
 190 memory to which the mapping refers, or if a smaller slab object could be used.
 191
 192 MREMAP_FIXED is not supported, though it is ignored if there's no change of
 193 address and the object does not need to be moved.
 194
 195 Shared mappings may not be moved.  Shareable mappings may not be moved either,
 196 even if they are not currently shared.
 197
 198 The mremap() function must be given an exact match for base address and size of
 199 a previously mapped object.  It may not be used to create holes in existing
 200 mappings, move parts of existing mappings or resize parts of mappings.  It must
 201 act on a complete mapping.
 202
 203 [*] Not currently supported.
 204
 205
 206 ============================================
 207 PROVIDING SHAREABLE CHARACTER DEVICE SUPPORT
 208 ============================================
 209
 210 To provide shareable character device support, a driver must provide a
 211 file->f_op->get_unmapped_area() operation. The mmap() routines will call this
 212 to get a proposed address for the mapping. This may return an error if it
 213 doesn't wish to honour the mapping because it's too long, at a weird offset,
 214 under some unsupported combination of flags or whatever.
 215
 216 The driver should also provide backing device information with capabilities set
 217 to indicate the permitted types of mapping on such devices. The default is
 218 assumed to be readable and writable, not executable, and only shareable
 219 directly (can't be copied).
 220
 221 The file->f_op->mmap() operation will be called to actually inaugurate the
 222 mapping. It can be rejected at that point. Returning the ENOSYS error will
 223 cause the mapping to be copied instead if BDI_CAP_MAP_COPY is specified.
 224
 225 The vm_ops->close() routine will be invoked when the last mapping on a chardev
 226 is removed. An existing mapping will be shared, partially or not, if possible
 227 without notifying the driver.
 228
 229 It is permitted also for the file->f_op->get_unmapped_area() operation to
 230 return -ENOSYS. This will be taken to mean that this operation just doesn't
 231 want to handle it, despite the fact it's got an operation. For instance, it
 232 might try directing the call to a secondary driver which turns out not to
 233 implement it. Such is the case for the framebuffer driver which attempts to
 234 direct the call to the device-specific driver. Under such circumstances, the
 235 mapping request will be rejected if BDI_CAP_MAP_COPY is not specified, and a
 236 copy mapped otherwise.
 237
 238 IMPORTANT NOTE:
 239
 240         Some types of device may present a different appearance to anyone
 241         looking at them in certain modes. Flash chips can be like this; for
 242         instance if they're in programming or erase mode, you might see the
 243         status reflected in the mapping, instead of the data.
 244
 245         In such a case, care must be taken lest userspace see a shared or a
 246         private mapping showing such information when the driver is busy
 247         controlling the device. Remember especially: private executable
 248         mappings may still be mapped directly off the device under some
 249         circumstances!
 250
 251
 252 ==============================================
 253 PROVIDING SHAREABLE MEMORY-BACKED FILE SUPPORT
 254 ==============================================
 255
 256 Provision of shared mappings on memory backed files is similar to the provision
 257 of support for shared mapped character devices. The main difference is that the
 258 filesystem providing the service will probably allocate a contiguous collection
 259 of pages and permit mappings to be made on that.
 260
 261 It is recommended that a truncate operation applied to such a file that
 262 increases the file size, if that file is empty, be taken as a request to gather
 263 enough pages to honour a mapping. This is required to support POSIX shared
 264 memory.
 265
 266 Memory backed devices are indicated by the mapping's backing device info having
 267 the memory_backed flag set.
 268
 269
 270 ========================================
 271 PROVIDING SHAREABLE BLOCK DEVICE SUPPORT
 272 ========================================
 273
 274 Provision of shared mappings on block device files is exactly the same as for
 275 character devices. If there isn't a real device underneath, then the driver
 276 should allocate sufficient contiguous memory to honour any supported mapping.
 277
 278
 279 =================================
 280 ADJUSTING PAGE TRIMMING BEHAVIOUR
 281 =================================
 282
 283 NOMMU mmap automatically rounds up to the nearest power-of-2 number of pages
 284 when performing an allocation.  This can have adverse effects on memory
 285 fragmentation, and as such, is left configurable.  The default behaviour is to
 286 aggressively trim allocations and discard any excess pages back in to the page
 287 allocator.  In order to retain finer-grained control over fragmentation, this
 288 behaviour can either be disabled completely, or bumped up to a higher page
 289 watermark where trimming begins.
 290
 291 Page trimming behaviour is configurable via the sysctl `vm.nr_trim_pages'.