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Subjectsyscall documentation
The note with the above title last week was very successful -
several people sent man pages. The copyright situation for
the *xattr pages is not entirely clear yet, but there is
good hope that it will be soon.

Let me send five new pages to l-k - maybe someone has
additions, corrections or comments.

The first one is alloc_hugepages.2. Below.
Probably more can be said about hugetlbfs.


alloc_hugepages, free_hugepages - allocate or free huge

void *alloc_hugepages(int key, void *addr, size_t len, int
prot, int flag);

int free_hugepages (void *addr);

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 CON­
FIG_HUGETLB_PAGE). In Linux 2.4.20 the syscall numbers
exist, but the calls return ENOSYS.

On i386 the memory management hardware knows about ordi­
nary 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 seg­
ment is created when none with the given key existed. If
this flag is not set, then ENOENT is returned when no seg­
ment 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.

ENOSYS The system call is not supported on this kernel.

This call is specific to Linux on Intel processors, and
should not be used in programs intended to be portable.
Indeed, the system call numbers are marked for reuse, so
programs using these may do something random on a future

/proc/sys/vm/nr_hugepages Number of configured hugetlb
pages. This can be read and written.

/proc/meminfo Gives info on the number of configured
hugetlb pages and on their size in the three variables
HugePages_Total, HugePages_Free, Hugepagesize.

The system calls are gone. Now the hugetlbfs filesystem
can be used instead. Memory backed by huge pages (if the
CPU supports them) is obtained by mmap'ing files in this
virtual filesystem.

The maximal number of huge pages can be specified using
the hugepages= boot parameter.

Linux 2.5.36 2003-02-02 ALLOC_HUGEPAGES(2)
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