1=============================
2No-MMU memory mapping support
3=============================
4
5The kernel has limited support for memory mapping under no-MMU conditions, such
6as are used in uClinux environments. From the userspace point of view, memory
7mapping is made use of in conjunction with the mmap() system call, the shmat()
8call and the execve() system call. From the kernel's point of view, execve()
9mapping is actually performed by the binfmt drivers, which call back into the
10mmap() routines to do the actual work.
11
12Memory mapping behaviour also involves the way fork(), vfork(), clone() and
13ptrace() work. Under uClinux there is no fork(), and clone() must be supplied
14the CLONE_VM flag.
15
16The behaviour is similar between the MMU and no-MMU cases, but not identical;
17and 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 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
108Further 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
163Interprocess shared memory
164==========================
165
166Both SYSV IPC SHM shared memory and POSIX shared memory is supported in NOMMU
167mode.  The former through the usual mechanism, the latter through files created
168on ramfs or tmpfs mounts.
169
170
171Futexes
172=======
173
174Futexes are supported in NOMMU mode if the arch supports them.  An error will
175be given if an address passed to the futex system call lies outside the
176mappings made by a process or if the mapping in which the address lies does not
177support futexes (such as an I/O chardev mapping).
178
179
180No-MMU mremap
181=============
182
183The mremap() function is partially supported.  It may change the size of a
184mapping, and may move it [#]_ if MREMAP_MAYMOVE is specified and if the new size
185of the mapping exceeds the size of the slab object currently occupied by the
186memory to which the mapping refers, or if a smaller slab object could be used.
187
188MREMAP_FIXED is not supported, though it is ignored if there's no change of
189address and the object does not need to be moved.
190
191Shared mappings may not be moved.  Shareable mappings may not be moved either,
192even if they are not currently shared.
193
194The mremap() function must be given an exact match for base address and size of
195a previously mapped object.  It may not be used to create holes in existing
196mappings, move parts of existing mappings or resize parts of mappings.  It must
197act on a complete mapping.
198
199.. [#] Not currently supported.
200
201
202Providing shareable character device support
203============================================
204
205To provide shareable character device support, a driver must provide a
206file->f_op->get_unmapped_area() operation. The mmap() routines will call this
207to get a proposed address for the mapping. This may return an error if it
208doesn't wish to honour the mapping because it's too long, at a weird offset,
209under some unsupported combination of flags or whatever.
210
211The driver should also provide backing device information with capabilities set
212to indicate the permitted types of mapping on such devices. The default is
213assumed to be readable and writable, not executable, and only shareable
214directly (can't be copied).
215
216The file->f_op->mmap() operation will be called to actually inaugurate the
217mapping. It can be rejected at that point. Returning the ENOSYS error will
218cause the mapping to be copied instead if NOMMU_MAP_COPY is specified.
219
220The vm_ops->close() routine will be invoked when the last mapping on a chardev
221is removed. An existing mapping will be shared, partially or not, if possible
222without notifying the driver.
223
224It is permitted also for the file->f_op->get_unmapped_area() operation to
225return -ENOSYS. This will be taken to mean that this operation just doesn't
226want to handle it, despite the fact it's got an operation. For instance, it
227might try directing the call to a secondary driver which turns out not to
228implement it. Such is the case for the framebuffer driver which attempts to
229direct the call to the device-specific driver. Under such circumstances, the
230mapping request will be rejected if NOMMU_MAP_COPY is not specified, and a
231copy 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
247Providing shareable memory-backed file support
248==============================================
249
250Provision of shared mappings on memory backed files is similar to the provision
251of support for shared mapped character devices. The main difference is that the
252filesystem providing the service will probably allocate a contiguous collection
253of pages and permit mappings to be made on that.
254
255It is recommended that a truncate operation applied to such a file that
256increases the file size, if that file is empty, be taken as a request to gather
257enough pages to honour a mapping. This is required to support POSIX shared
258memory.
259
260Memory backed devices are indicated by the mapping's backing device info having
261the memory_backed flag set.
262
263
264Providing shareable block device support
265========================================
266
267Provision of shared mappings on block device files is exactly the same as for
268character devices. If there isn't a real device underneath, then the driver
269should allocate sufficient contiguous memory to honour any supported mapping.
270
271
272Adjusting page trimming behaviour
273=================================
274
275NOMMU mmap automatically rounds up to the nearest power-of-2 number of pages
276when performing an allocation.  This can have adverse effects on memory
277fragmentation, and as such, is left configurable.  The default behaviour is to
278aggressively trim allocations and discard any excess pages back in to the page
279allocator.  In order to retain finer-grained control over fragmentation, this
280behaviour can either be disabled completely, or bumped up to a higher page
281watermark where trimming begins.
282
283Page trimming behaviour is configurable via the sysctl ``vm.nr_trim_pages``.
284