- =============================
- NO-MMU MEMORY MAPPING SUPPORT
- =============================
+=============================
+No-MMU memory mapping support
+=============================
The kernel has limited support for memory mapping under no-MMU conditions, such
as are used in uClinux environments. From the userspace point of view, memory
The behaviour is similar between the MMU and no-MMU cases, but not identical;
and it's also much more restricted in the latter case:
- (*) Anonymous mapping, MAP_PRIVATE
+ (#) Anonymous mapping, MAP_PRIVATE
In the MMU case: VM regions backed by arbitrary pages; copy-on-write
across fork.
In the no-MMU case: VM regions backed by arbitrary contiguous runs of
pages.
- (*) Anonymous mapping, MAP_SHARED
+ (#) Anonymous mapping, MAP_SHARED
These behave very much like private mappings, except that they're
shared across fork() or clone() without CLONE_VM in the MMU case. Since
the no-MMU case doesn't support these, behaviour is identical to
MAP_PRIVATE there.
- (*) File, MAP_PRIVATE, PROT_READ / PROT_EXEC, !PROT_WRITE
+ (#) File, MAP_PRIVATE, PROT_READ / PROT_EXEC, !PROT_WRITE
In the MMU case: VM regions backed by pages read from file; changes to
the underlying file are reflected in the mapping; copied across fork.
are visible in other processes (no MMU protection), but should not
happen.
- (*) File, MAP_PRIVATE, PROT_READ / PROT_EXEC, PROT_WRITE
+ (#) File, MAP_PRIVATE, PROT_READ / PROT_EXEC, PROT_WRITE
In the MMU case: like the non-PROT_WRITE case, except that the pages in
question get copied before the write actually happens. From that point
In the no-MMU case: works much like the non-PROT_WRITE case, except
that a copy is always taken and never shared.
- (*) Regular file / blockdev, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
+ (#) Regular file / blockdev, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
In the MMU case: VM regions backed by pages read from file; changes to
pages written back to file; writes to file reflected into pages backing
In the no-MMU case: not supported.
- (*) Memory backed regular file, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
+ (#) Memory backed regular file, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
In the MMU case: As for ordinary regular files.
as for the MMU case. If the filesystem does not provide any such
support, then the mapping request will be denied.
- (*) Memory backed blockdev, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
+ (#) Memory backed blockdev, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
In the MMU case: As for ordinary regular files.
truncate being called. The ramdisk driver could do this if it allocated
all its memory as a contiguous array upfront.
- (*) Memory backed chardev, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
+ (#) Memory backed chardev, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
In the MMU case: As for ordinary regular files.
provide any such support, then the mapping request will be denied.
-============================
-FURTHER NOTES ON NO-MMU MMAP
+Further notes on no-MMU MMAP
============================
- (*) A request for a private mapping of a file may return a buffer that is not
+ (#) A request for a private mapping of a file may return a buffer that is not
page-aligned. This is because XIP may take place, and the data may not be
paged aligned in the backing store.
- (*) A request for an anonymous mapping will always be page aligned. If
+ (#) A request for an anonymous mapping will always be page aligned. If
possible the size of the request should be a power of two otherwise some
of the space may be wasted as the kernel must allocate a power-of-2
granule but will only discard the excess if appropriately configured as
this has an effect on fragmentation.
- (*) The memory allocated by a request for an anonymous mapping will normally
+ (#) The memory allocated by a request for an anonymous mapping will normally
be cleared by the kernel before being returned in accordance with the
Linux man pages (ver 2.22 or later).
uClibc uses this to speed up malloc(), and the ELF-FDPIC binfmt uses this
to allocate the brk and stack region.
- (*) A list of all the private copy and anonymous mappings on the system is
+ (#) A list of all the private copy and anonymous mappings on the system is
visible through /proc/maps in no-MMU mode.
- (*) A list of all the mappings in use by a process is visible through
+ (#) A list of all the mappings in use by a process is visible through
/proc/<pid>/maps in no-MMU mode.
- (*) Supplying MAP_FIXED or a requesting a particular mapping address will
+ (#) Supplying MAP_FIXED or a requesting a particular mapping address will
result in an error.
- (*) Files mapped privately usually have to have a read method provided by the
+ (#) Files mapped privately usually have to have a read method provided by the
driver or filesystem so that the contents can be read into the memory
allocated if mmap() chooses not to map the backing device directly. An
error will result if they don't. This is most likely to be encountered
with character device files, pipes, fifos and sockets.
-==========================
-INTERPROCESS SHARED MEMORY
+Interprocess shared memory
==========================
Both SYSV IPC SHM shared memory and POSIX shared memory is supported in NOMMU
on ramfs or tmpfs mounts.
-=======
-FUTEXES
+Futexes
=======
Futexes are supported in NOMMU mode if the arch supports them. An error will
support futexes (such as an I/O chardev mapping).
-=============
-NO-MMU MREMAP
+No-MMU mremap
=============
The mremap() function is partially supported. It may change the size of a
-mapping, and may move it[*] if MREMAP_MAYMOVE is specified and if the new size
+mapping, and may move it [#]_ if MREMAP_MAYMOVE is specified and if the new size
of the mapping exceeds the size of the slab object currently occupied by the
memory to which the mapping refers, or if a smaller slab object could be used.
mappings, move parts of existing mappings or resize parts of mappings. It must
act on a complete mapping.
-[*] Not currently supported.
+.. [#] Not currently supported.
-============================================
-PROVIDING SHAREABLE CHARACTER DEVICE SUPPORT
+Providing shareable character device support
============================================
To provide shareable character device support, a driver must provide a
mapping request will be rejected if NOMMU_MAP_COPY is not specified, and a
copy mapped otherwise.
-IMPORTANT NOTE:
+.. important::
Some types of device may present a different appearance to anyone
looking at them in certain modes. Flash chips can be like this; for
circumstances!
-==============================================
-PROVIDING SHAREABLE MEMORY-BACKED FILE SUPPORT
+Providing shareable memory-backed file support
==============================================
Provision of shared mappings on memory backed files is similar to the provision
the memory_backed flag set.
-========================================
-PROVIDING SHAREABLE BLOCK DEVICE SUPPORT
+Providing shareable block device support
========================================
Provision of shared mappings on block device files is exactly the same as for
should allocate sufficient contiguous memory to honour any supported mapping.
-=================================
-ADJUSTING PAGE TRIMMING BEHAVIOUR
+Adjusting page trimming behaviour
=================================
NOMMU mmap automatically rounds up to the nearest power-of-2 number of pages
behaviour can either be disabled completely, or bumped up to a higher page
watermark where trimming begins.
-Page trimming behaviour is configurable via the sysctl `vm.nr_trim_pages'.
+Page trimming behaviour is configurable via the sysctl ``vm.nr_trim_pages``.
git.samba.org / sfrench / cifs-2.6.git / blobdiff