--- /dev/null
+ =============================
+ 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
+mapping is made use of in conjunction with the mmap() system call, the shmat()
+call and the execve() system call. From the kernel's point of view, execve()
+mapping is actually performed by the binfmt drivers, which call back into the
+mmap() routines to do the actual work.
+
+Memory mapping behaviour also involves the way fork(), vfork(), clone() and
+ptrace() work. Under uClinux there is no fork(), and clone() must be supplied
+the CLONE_VM flag.
+
+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
+
+ 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
+
+ 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
+
+ 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.
+
+ In the no-MMU case: VM regions backed by arbitrary contiguous runs of
+ pages into which the appropriate bit of the file is read; any remaining
+ bit of the mapping is cleared; such mappings are shared if possible;
+ writes to the file do not affect the mapping; writes to the mapping are
+ visible in other processes (no MMU protection), but should not happen.
+
+ (*) 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
+ on writes to that page in the file no longer get reflected into the
+ mapping's backing pages.
+
+ In the no-MMU case: works exactly as for the non-PROT_WRITE case.
+
+ (*) 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
+ mapping; shared across fork.
+
+ In the no-MMU case: not supported.
+
+ (*) Memory backed regular file, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
+
+ In the MMU case: As for ordinary regular files.
+
+ In the no-MMU case: The filesystem providing the memory-backed file
+ (such as ramfs or tmpfs) may choose to honour an open, truncate, mmap
+ sequence by providing a contiguous sequence of pages to map. In that
+ case, a shared-writable memory mapping will be possible. It will work
+ as for the MMU case. If the filesystem does not provide any such
+ support, then the mapping request will be denied.
+
+ (*) Memory backed chardev, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
+
+ In the MMU case: As for ordinary regular files.
+
+ In the no-MMU case: The character device driver may choose to honour
+ the mmap() by providing direct access to the underlying device if it
+ provides memory or quasi-memory that can be accessed directly. Examples
+ of such are frame buffers and flash devices. If the driver does not
+ provide any such support, then the mapping request will be denied.
+
+
+============================
+FURTHER NOTES ON NO-MMU MMAP
+============================
+
+ (*) A request for a private mapping of less than a page in size may not return
+ a page-aligned buffer. This is because the kernel calls kmalloc() to
+ allocate the buffer, not get_free_page().
+
+ (*) A list of all the mappings on the system is visible through /proc/maps in
+ no-MMU mode.
+
+ (*) Supplying MAP_FIXED or a requesting a particular mapping address will
+ result in an error.
+
+ (*) Files mapped privately must have a read method provided by the driver or
+ filesystem so that the contents can be read into the memory allocated. An
+ error will result if they don't. This is most likely to be encountered
+ with character device files, pipes, fifos and sockets.
+
+
+============================================
+PROVIDING SHAREABLE CHARACTER DEVICE SUPPORT
+============================================
+
+To provide shareable character device support, a driver must provide a
+file->f_op->get_unmapped_area() operation. The mmap() routines will call this
+to get a proposed address for the mapping. This may return an error if it
+doesn't wish to honour the mapping because it's too long, at a weird offset,
+under some unsupported combination of flags or whatever.
+
+The vm_ops->close() routine will be invoked when the last mapping on a chardev
+is removed. An existing mapping will be shared, partially or not, if possible
+without notifying the driver.
+
+It is permitted also for the file->f_op->get_unmapped_area() operation to
+return -ENOSYS. This will be taken to mean that this operation just doesn't
+want to handle it, despite the fact it's got an operation. For instance, it
+might try directing the call to a secondary driver which turns out not to
+implement it. Such is the case for the framebuffer driver which attempts to
+direct the call to the device-specific driver.
+
+
+==============================================
+PROVIDING SHAREABLE MEMORY-BACKED FILE SUPPORT
+==============================================
+
+Provision of shared mappings on memory backed files is similar to the provision
+of support for shared mapped character devices. The main difference is that the
+filesystem providing the service will probably allocate a contiguous collection
+of pages and permit mappings to be made on that.
+
+It is recommended that a truncate operation applied to such a file that
+increases the file size, if that file is empty, be taken as a request to gather
+enough pages to honour a mapping. This is required to support POSIX shared
+memory.
+
+Memory backed devices are indicated by the mapping's backing device info having
+the memory_backed flag set.
git://git.onelab.eu / linux-2.6.git / blobdiff