[PATCH] fix remap of shared read only mappings

When mmap MAP_SHARED is done on a file, it gets marked with VM_MAYSHARE
and, if it's read/write, VM_SHARED.  However, if it is remapped with
mremap(), the MAP_SHARED is only passed into the new mapping based on
VM_SHARED.  This means that remapped read only MAP_SHARED mappings lose
VM_MAYSHARE.  This is causing us a problem on parisc because we have to
align all shared mappings carefully to mitigate cache aliasing problems.

The fix is to key passing the MAP_SHARED flag back into the remapped are
off VM_MAYSHARE not VM_SHARED.

mm/mremap.c

@@ -420,7 +420,7 @@ unsigned long do_mremap(unsigned long addr,
 	if (flags & MREMAP_MAYMOVE) {
 		if (!(flags & MREMAP_FIXED)) {
 			unsigned long map_flags = 0;
-			if (vma->vm_flags & VM_SHARED)
+			if (vma->vm_flags & VM_MAYSHARE)
 				map_flags |= MAP_SHARED;
 
 			new_addr = get_unmapped_area(vma->vm_file, 0, new_len,