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Documentation / nommu-mmap.txt




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Based on kernel version 3.4. Page generated on 2012-05-21 22:09 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 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'.
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