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Linux/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 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 ============================
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 NOMMU_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 NOMMU_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'.

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