The attached patch makes ramfs support shared-writable mmaps by:

 (1) Attempting to perform a contiguous block allocation to the requested size
     when truncate attempts to increase the file from zero size, such as
     happens when:

	fd = shm_open("/file/on/ramfs", ...):
	ftruncate(fd, size_requested);
	addr = mmap(NULL, subsize, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_SHARED,
		    fd, offset);

 (2) Permitting any shared-writable mapping over any contiguous set of extant
     pages. get_unmapped_area() will return the address into the actual ramfs
     pages. The mapping may start anywhere and be of any size, but may not go
     over the end of file. Multiple mappings may overlap in any way.

 (3) Not permitting a file to be shrunk if it would truncate any shared
     mappings (private mappings are copied).

Thus this patch provides support for POSIX shared memory on NOMMU kernels,
with certain limitations such as there being a large enough block of pages
available to support the allocation and it only working on directly mappable
filesystems.
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>