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CentOS 7.0 - man page for free_hugepages (centos section 2)

ALLOC_HUGEPAGES(2)		    Linux Programmer's Manual		       ALLOC_HUGEPAGES(2)

NAME
       alloc_hugepages, free_hugepages - allocate or free huge pages

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

       int free_hugepages(void *addr);

DESCRIPTION
       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 fail with the
       error 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's memory or to free them again.  Huge  pages  are
       locked into memory, and are not swapped.

       The  key argument 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  argument 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 argument of alloc_hugepages() is a hint, that the kernel may
       or may not follow.  Addresses must be properly aligned.

       The len argument is the length of the required segment.	It must be a multiple of the huge
       page size.

       The prot argument specifies the memory protection of the segment.  It is one of PROT_READ,
       PROT_WRITE, PROT_EXEC.

       The flag argument 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.

RETURN VALUE
       On success, alloc_hugepages() returns the allocated virtual address, and  free_hugepages()
       returns zero.  On error, -1 is returned, and errno is set appropriately.

ERRORS
       ENOSYS The system call is not supported on this kernel.

FILES
       /proc/sys/vm/nr_hugepages  Number of configured hugetlb pages.  This can be read and writ-
       ten.

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

CONFORMING TO
       These  calls are specific to Linux on Intel processors, and should not be used in programs
       intended to be portable.

NOTES
       These system calls are gone; they existed only in Linux 2.5.36 through to 2.5.54.  Now the
       hugetlbfs  file	system can be used instead.  Memory backed by huge pages (if the CPU sup-
       ports them) is obtained by using mmap(2) to map files in this virtual file system.

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

COLOPHON
       This page is part of release 3.53 of the Linux man-pages project.  A  description  of  the
       project,     and    information	  about    reporting	bugs,	 can	be    found    at
       http://www.kernel.org/doc/man-pages/.

Linux					    2007-05-31			       ALLOC_HUGEPAGES(2)


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