[PATCH] NOMMU: Provide shared-writable mmap support on ramfs

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>

1 files changed, 10 insertions, 0 deletions

diff --git a/include/linux/ramfs.h b/include/linux/ramfs.h
index e0a4faa..953b6df 100644
--- a/include/linux/ramfs.h
+++ b/include/linux/ramfs.h
@@ -5,6 +5,16 @@ struct inode *ramfs_get_inode(struct super_block *sb, int mode, dev_t dev);
 struct super_block *ramfs_get_sb(struct file_system_type *fs_type,
 	 int flags, const char *dev_name, void *data);
 
+#ifndef CONFIG_MMU
+extern unsigned long ramfs_nommu_get_unmapped_area(struct file *file,
+						   unsigned long addr,
+						   unsigned long len,
+						   unsigned long pgoff,
+						   unsigned long flags);
+
+extern int ramfs_nommu_mmap(struct file *file, struct vm_area_struct *vma);
+#endif
+
 extern struct file_operations ramfs_file_operations;
 extern struct vm_operations_struct generic_file_vm_ops;