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Documentation / nommu-mmap.txt

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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

1	=============================
2	No-MMU memory mapping support
3	=============================
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.
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.
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:
19	 (#) Anonymous mapping, MAP_PRIVATE
21		In the MMU case: VM regions backed by arbitrary pages; copy-on-write
22		across fork.
24		In the no-MMU case: VM regions backed by arbitrary contiguous runs of
25		pages.
27	 (#) Anonymous mapping, MAP_SHARED
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.
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.
39		In the no-MMU case:
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.
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.
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
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.
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.
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.
69	 (#) Regular file / blockdev, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
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.
75		In the no-MMU case: not supported.
77	 (#) Memory backed regular file, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
79		In the MMU case: As for ordinary regular files.
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.
88	 (#) Memory backed blockdev, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
90		In the MMU case: As for ordinary regular files.
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.
97	 (#) Memory backed chardev, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
99		In the MMU case: As for ordinary regular files.
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.
108	Further notes on no-MMU MMAP
109	============================
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.
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.
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).
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.
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.
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.
144	     uClibc uses this to speed up malloc(), and the ELF-FDPIC binfmt uses this
145	     to allocate the brk and stack region.
147	 (#) A list of all the private copy and anonymous mappings on the system is
148	     visible through /proc/maps in no-MMU mode.
150	 (#) A list of all the mappings in use by a process is visible through
151	     /proc/<pid>/maps in no-MMU mode.
153	 (#) Supplying MAP_FIXED or a requesting a particular mapping address will
154	     result in an error.
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.
163	Interprocess shared memory
164	==========================
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.
171	Futexes
172	=======
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).
180	No-MMU mremap
181	=============
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.
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.
191	Shared mappings may not be moved.  Shareable mappings may not be moved either,
192	even if they are not currently shared.
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.
199	.. [#] Not currently supported.
202	Providing shareable character device support
203	============================================
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.
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).
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.
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.
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.
233	.. important::
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.
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!
247	Providing shareable memory-backed file support
248	==============================================
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.
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.
260	Memory backed devices are indicated by the mapping's backing device info having
261	the memory_backed flag set.
264	Providing shareable block device support
265	========================================
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.
272	Adjusting page trimming behaviour
273	=================================
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.
283	Page trimming behaviour is configurable via the sysctl ``vm.nr_trim_pages``.
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