• David Howells's avatar
    [PATCH] NOMMU: Provide shared-writable mmap support on ramfs · 642fb4d1
    David Howells authored
    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: default avatarDavid Howells <dhowells@redhat.com>
    Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
    642fb4d1
file-nommu.c 7.43 KB