The system calls alloc_hugepages and free_hugepages were introduced in Linux 2.5.36 and removed again in 2.5.54. They existed only on i386 and ia64 (when built with CONFIG_HUGETLB_PAGE). In Linux 2.4.20 the syscall numbers exist, but the calls return ENOSYS.
On i386 the memory management hardware knows about ordinary pages (4 KiB) and huge pages (2 or 4 MiB). Similarly ia64 knows about huge pages of several sizes. These system calls serve to map huge pages into the process memory or to free them again. Huge pages are locked into memory, and are not swapped.
The key parameter is an identifier. When zero the pages are private, and not inherited by children. When positive the pages are shared with other applications using the same key, and inherited by child processes.
The addr parameter of free_hugepages() tells which page is being freed - it was the return value of a call to alloc_hugepages(). (The memory is first actually freed when all users have released it.) The addr parameter of alloc_hugepages() is a hint, that the kernel may or may not follow. Addresses must be properly aligned.
The len parameter is the length of the required segment. It must be a multiple of the huge page size.
The prot parameter specifies the memory protection of the segment. It is one of PROT_READ, PROT_WRITE, PROT_EXEC.
The flag parameter is ignored, unless key is positive. In that case, if flag is IPC_CREAT, then a new huge page segment is created when none with the given key existed. If this flag is not set, then ENOENT is returned when no segment with the given key exists.
On success, alloc_hugepages returns the allocated virtual address, and free_hugepages returns zero. On error, -1 is returned, and errno is set appropriately.