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
6 as are used in uClinux environments. From the userspace point of view, memory
12 Memory mapping behaviour also involves the way fork(), vfork(), clone() and
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
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
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,
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
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
67 	that a copy is always taken and never shared.
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
81 	In the no-MMU case: The filesystem providing the memory-backed file
84 	case, a shared-writable memory mapping will be possible. It will work
88  (#) Memory backed blockdev, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
92 	In the no-MMU case: As for memory backed regular files, but the
95 	all its memory as a contiguous array upfront.
97  (#) Memory backed chardev, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
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
121  (#) The memory allocated by a request for an anonymous mapping will normally
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
140      indicate to the kernel that it shouldn't bother clearing the memory before
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.
157      driver or filesystem so that the contents can be read into the memory
163 Interprocess shared memory
166 Both SYSV IPC SHM shared memory and POSIX shared memory is supported in NOMMU
180 No-MMU mremap
186 memory to which the mapping refers, or if a smaller slab object could be used.
191 Shared mappings may not be moved.  Shareable mappings may not be moved either,
192 even if they are not currently shared.
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
221 is removed. An existing mapping will be shared, partially or not, if possible
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
240 	In such a case, care must be taken lest userspace see a shared or a
247 Providing shareable memory-backed file support
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
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
267 Provision of shared mappings on block device files is exactly the same as for
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
276 when performing an allocation.  This can have adverse effects on memory
279 allocator.  In order to retain finer-grained control over fragmentation, this