mirror of https://github.com/mkerrisk/man-pages
155 lines
4.6 KiB
Groff
155 lines
4.6 KiB
Groff
.\" Copyright 2003 Andries E. Brouwer (aeb@cwi.nl)
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.TH ALLOC_HUGEPAGES 2 2021-03-22 "Linux" "Linux Programmer's Manual"
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.SH NAME
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alloc_hugepages, free_hugepages \- allocate or free huge pages
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.SH SYNOPSIS
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.nf
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.BI "void *syscall(SYS_alloc_hugepages, int " key ", void *" addr \
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", size_t " len ,
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.BI " int " prot ", int " flag );
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.\" asmlinkage unsigned long sys_alloc_hugepages(int key, unsigned long addr,
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.\" unsigned long len, int prot, int flag);
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.BI "int syscall(SYS_free_hugepages, void *" addr );
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.\" asmlinkage int sys_free_hugepages(unsigned long addr);
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.fi
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.PP
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.IR Note :
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glibc provides no wrappers for these system calls,
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necessitating the use of
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.BR syscall (2).
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.SH DESCRIPTION
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The system calls
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.BR alloc_hugepages ()
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and
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.BR free_hugepages ()
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were introduced in Linux 2.5.36 and removed again in 2.5.54.
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They existed only on i386 and ia64 (when built with
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.BR CONFIG_HUGETLB_PAGE ).
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In Linux 2.4.20, the syscall numbers exist,
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but the calls fail with the error
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.BR ENOSYS .
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.PP
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On i386 the memory management hardware knows about ordinary pages (4\ KiB)
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and huge pages (2 or 4\ MiB).
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Similarly ia64 knows about huge pages of
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several sizes.
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These system calls serve to map huge pages into the
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process's memory or to free them again.
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Huge pages are locked into memory, and are not swapped.
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.PP
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The
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.I key
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argument is an identifier.
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When zero the pages are private, and
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not inherited by children.
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When positive the pages are shared with other applications using the same
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.IR key ,
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and inherited by child processes.
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.PP
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The
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.I addr
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argument of
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.BR free_hugepages ()
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tells which page is being freed: it was the return value of a
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call to
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.BR alloc_hugepages ().
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(The memory is first actually freed when all users have released it.)
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The
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.I addr
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argument of
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.BR alloc_hugepages ()
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is a hint, that the kernel may or may not follow.
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Addresses must be properly aligned.
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.PP
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The
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.I len
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argument is the length of the required segment.
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It must be a multiple of the huge page size.
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.PP
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The
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.I prot
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argument specifies the memory protection of the segment.
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It is one of
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.BR PROT_READ ,
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.BR PROT_WRITE ,
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.BR PROT_EXEC .
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.PP
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The
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.I flag
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argument is ignored, unless
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.I key
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is positive.
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In that case, if
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.I flag
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is
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.BR IPC_CREAT ,
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then a new huge page segment is created when none
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with the given key existed.
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If this flag is not set, then
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.B ENOENT
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is returned when no segment with the given key exists.
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.SH RETURN VALUE
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On success,
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.BR alloc_hugepages ()
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returns the allocated virtual address, and
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.BR free_hugepages ()
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returns zero.
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On error, \-1 is returned, and
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.I errno
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is set to indicate the error.
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.SH ERRORS
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.TP
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.B ENOSYS
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The system call is not supported on this kernel.
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.SH FILES
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.TP
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.I /proc/sys/vm/nr_hugepages
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Number of configured hugetlb pages.
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This can be read and written.
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.TP
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.I /proc/meminfo
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Gives info on the number of configured hugetlb pages and on their size
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in the three variables HugePages_Total, HugePages_Free, Hugepagesize.
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.SH CONFORMING TO
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These extinct system calls were specific to Linux on Intel processors.
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.SH NOTES
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These system calls are gone;
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they existed only in Linux 2.5.36 through to 2.5.54.
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Now the hugetlbfs filesystem can be used instead.
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Memory backed by huge pages (if the CPU supports them) is obtained by
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using
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.BR mmap (2)
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to map files in this virtual filesystem.
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.PP
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The maximal number of huge pages can be specified using the
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.B hugepages=
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boot parameter.
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.\".PP
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.\" requires CONFIG_HUGETLB_PAGE (under "Processor type and features")
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.\" and CONFIG_HUGETLBFS (under "Filesystems").
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.\" mount \-t hugetlbfs hugetlbfs /huge
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.\" SHM_HUGETLB
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