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
16 The behaviour is similar between the MMU and no-MMU cases, but not identical;
21 	In the MMU case: VM regions backed by arbitrary pages; copy-on-write
24 	In the no-MMU case: VM regions backed by arbitrary contiguous runs of
31 	the no-MMU case doesn't support these, behaviour is identical to
39 	In the no-MMU case:
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
50 	 - If the backing device can't or won't permit direct sharing,
52            appropriate bit of the file will be read into a contiguous bit of
55 	 - Writes to the file do not affect the mapping; writes to the mapping
61 	In the MMU case: like the non-PROT_WRITE case, except that the pages in
66 	In the no-MMU case: works much like the non-PROT_WRITE case, except
75 	In the no-MMU case: not supported.
81 	In the no-MMU case: The filesystem providing the memory-backed file
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
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
95 	all its memory as a contiguous array upfront.
101 	In the no-MMU case: The character device driver may choose to honour
103 	provides memory or quasi-memory that can be accessed directly. Examples
108 Further notes on no-MMU MMAP
112      page-aligned.  This is because XIP may take place, and the data may not be
117      of the space may be wasted as the kernel must allocate a power-of-2
130      initialize the contents of a page - depending on the write-usage of the
133      In the no-MMU case, however, anonymous mappings are backed by physical
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
144      uClibc uses this to speed up malloc(), and the ELF-FDPIC binfmt uses this
148      visible through /proc/maps in no-MMU mode.
151      /proc/<pid>/maps in no-MMU mode.
180 No-MMU mremap
206 file->f_op->get_unmapped_area() operation. The mmap() routines will call this
216 The file->f_op->mmap() operation will be called to actually inaugurate the
220 The vm_ops->close() routine will be invoked when the last mapping on a chardev
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
229 direct the call to the device-specific driver. Under such circumstances, the
247 Providing shareable memory-backed file support
252 filesystem providing the service will probably allocate a contiguous collection
269 should allocate sufficient contiguous memory to honour any supported mapping.
275 NOMMU mmap automatically rounds up to the nearest power-of-2 number of pages
279 allocator.  In order to retain finer-grained control over fragmentation, this