admin-guide/mm/nommu-mmap.rst

2 No-MMU memory mapping support
5 The kernel has limited support for memory mapping under no-MMU conditions, such
7 mapping is made use of in conjunction with the mmap() system call, the shmat()
9 mapping is actually performed by the binfmt drivers, which call back into the
12 Memory mapping behaviour also involves the way fork(), vfork(), clone() and
19  (#) Anonymous mapping, MAP_PRIVATE
27  (#) Anonymous mapping, MAP_SHARED
37 	the underlying file are reflected in the mapping; copied across fork.
41          - If one exists, the kernel will re-use an existing mapping to the
45          - If possible, the file mapping will be directly on the backing device
47            appropriate mapping protection capabilities. Ramfs, romfs, cramfs
55 	 - Writes to the file do not affect the mapping; writes to the mapping
64 	the mapping's backing pages. The page is then backed by swap instead.
73 	mapping; shared across fork.
84 	case, a shared-writable memory mapping will be possible. It will work
86 	support, then the mapping request will be denied.
105 	provide any such support, then the mapping request will be denied.
111  (#) A request for a private mapping of a file may return a buffer that is not
115  (#) A request for an anonymous mapping will always be page aligned.  If
121  (#) The memory allocated by a request for an anonymous mapping will normally
131      mapping.
136      anonymous mapping and the kernel then does a memset for the entire map.
153  (#) Supplying MAP_FIXED or a requesting a particular mapping address will
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).
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.
197 act on a complete mapping.
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,
212 to indicate the permitted types of mapping on such devices. The default is
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
230 mapping request will be rejected if NOMMU_MAP_COPY is not specified, and a
238 	status reflected in the mapping, instead of the data.
241 	private mapping showing such information when the driver is busy
257 enough pages to honour a mapping. This is required to support POSIX shared
260 Memory backed devices are indicated by the mapping's backing device info having
269 should allocate sufficient contiguous memory to honour any supported mapping.