.\" Copyright (c) 2010 Michael Kerrisk <mtk.manpages@gmail.com>
.\" based on a proposal from Stephan Mueller <smueller@atsec.com>
.\"
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.\" preserved on all copies.
.\" a permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
-.\" manual page may be incorrect or out-of-date. The author(s) assume.
-.\" no responsibility for errors or omissions, or for damages resulting.
-.\" from the use of the information contained herein. The author(s) may.
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+.\" from the use of the information contained herein. The author(s) may
+.\" not have taken the same level of care in the production of this
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.\"
.\" Various pieces of text taken from the kernel source and the commentary
.\" in kernel commit fa28237cfcc5827553044cbd6ee52e33692b0faa
.\" both written by Paul Mackerras <paulus@samba.org>
.\"
-.TH SUBPAGE_PROT 2 2010-10-09 "Linux" "Linux Programmer's Manual"
+.TH SUBPAGE_PROT 2 2019-03-06 "Linux" "Linux Programmer's Manual"
.SH NAME
-subpage_prot \- copy a subpage protection map into the kernel
+subpage_prot \- define a subpage protection for an address range
.SH SYNOPSIS
.nf
.BI "long subpage_prot(unsigned long " addr ", unsigned long " len ,
.BI " uint32_t *" map ");
.fi
+.PP
+.IR Note :
+There is no glibc wrapper for this system call; see NOTES.
.SH DESCRIPTION
The PowerPC-specific
.BR subpage_prot ()
system call provides the facility to control the access
-permissions on individual 4kB subpages on systems configured with
-a page size of 64kB.
-
+permissions on individual 4\ kB subpages on systems configured with
+a page size of 64\ kB.
+.PP
The protection map is applied to the memory pages in the region starting at
.I addr
and continuing for
.I len
bytes.
Both of these arguments must be aligned to a 64-kB boundary.
-
+.PP
The protection map is specified in the buffer pointed to by
.IR map .
-The map has 2 bits per 4kB subpage;
-thus each 32-bit word specifies the protections of 16 4kB subpages
-inside a 64kB page
+The map has 2 bits per 4\ kB subpage;
+thus each 32-bit word specifies the protections of 16 4\ kB subpages
+inside a 64\ kB page
(so, the number of 32-bit words pointed to by
.I map
should equate to the number of 64-kB pages specified by
On success,
.BR subpage_prot ()
returns 0.
-Otherwise, one of the negated error codes specified below is returned.
+Otherwise, one of the error codes specified below is returned.
.SH ERRORS
.TP
+.B EFAULT
+The buffer referred to by
+.I map
+is not accessible.
+.TP
.B EINVAL
The
.I addr
and they must not refer to a region outside of the
address space of the process or to a region that consists of huge pages.
.TP
-.B EFAULT
-The buffer referred to by
-.I map
-is not accessible.
-.TP
.B ENOMEM
Out of memory.
.SH VERSIONS
.SH CONFORMING TO
This system call is Linux-specific.
.SH NOTES
+Glibc does not provide a wrapper for this system call; call it using
+.BR syscall (2).
+.PP
Normal page protections (at the 64-kB page level) also apply;
the subpage protection mechanism is an additional constraint,
so putting 0 in a 2-bit field won't allow writes to a page that is otherwise
These masks are applied at the level where hardware page-table entries (PTEs)
are inserted into the hardware page table based on the Linux PTEs,
so the Linux PTEs are not affected.
-.\" Perhaps we don't need to document this implementation detail:
-.\"
-.\" Implicit in this is that the regions of the address space that are
-.\" protected are switched to use 4k hardware pages rather than 64k
-.\" hardware pages (on machines with hardware 64k page support).
-.\" In fact the whole process is switched to use 4k hardware pages when the
-.\" subpage_prot system call is used, but this could be improved in future
-.\" to switch only the affected segments.
+Implicit in this is that the regions of the address space that are
+protected are switched to use 4-kB hardware pages rather than 64-kB
+hardware pages (on machines with hardware 64-kB page support).
+.\" In the initial implementation, it was the case that:
+.\" In fact the whole process is switched to use 4 kB hardware pages when the
+.\" subpage_prot system call is used, but this could be improved in future
+.\" to switch only the affected segments.
+.\" But Paul Mackerass says (Oct 2010): I'm pretty sure we now only switch
+.\" the affected segment, not the whole process.
.SH SEE ALSO
.BR mprotect (2),
-.BR syscall (2);
-.br
-the kernel source file
-.IR Documentation/vm/hugetlbpage.txt .
+.BR syscall (2)
+.PP
+.IR Documentation/admin-guide/mm/hugetlbpage.rst
+in the Linux kernel source tree
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