man-pages/man2/move_pages.2

.\" This manpage is Copyright (C) 2006 Silicon Graphics, Inc.
.\"                               Christoph Lameter
.\"
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.\" FIXME Should programs normally be using move_pages() directly, or should
.\" they rather be using interfaces in the numactl package?
.\" (e.g., compare with recommendation in mbind(2)).
.\" Does this page need to give advice on this topic?
.\"
.TH MOVE_PAGES 2 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
move_pages \- move individual pages of a process to another node
.SH SYNOPSIS
.nf
.B #include <numaif.h>
.PP
.BI "long move_pages(int " pid ", unsigned long " count ", void **" pages ,
.BI "                const int *" nodes ", int *" status ", int " flags );
.fi
.PP
Link with \fI\-lnuma\fP.
.PP
.IR Note :
There is no glibc wrapper for this system call; see NOTES.
.SH DESCRIPTION
.BR move_pages ()
moves the specified
.I pages
of the process
.I pid
to the memory nodes specified by
.IR nodes .
The result of the move is reflected in
.IR status .
The
.I flags
indicate constraints on the pages to be moved.
.PP
.I pid
is the ID of the process in which pages are to be moved.
If
.I pid
is 0, then
.BR move_pages ()
moves pages of the calling process.
.PP
To move pages in another process requires the following privileges:
.IP * 3
In kernels up to and including Linux 4.12:
the caller must be privileged
.RB ( CAP_SYS_NICE )
or the real or effective user ID of the calling process must match the
real or saved-set user ID of the target process.
.IP *
The older rules allowed the caller to discover various
virtual address choices made by the kernel that could lead
to the defeat of address-space-layout randomization
for a process owned by the same UID as the caller,
the rules were changed starting with Linux 4.13.
Since Linux 4.13,
.\" commit 197e7e521384a23b9e585178f3f11c9fa08274b9
permission is governed by a ptrace access mode
.B PTRACE_MODE_READ_REALCREDS
check with respect to the target process; see
.BR ptrace (2).
.PP
.I count
is the number of pages to move.
It defines the size of the three arrays
.IR pages ,
.IR nodes ,
and
.IR status .
.PP
.I pages
is an array of pointers to the pages that should be moved.
These are pointers that should be aligned to page boundaries.
.\" FIXME Describe the result if pointers in the 'pages' array are
.\" not aligned to page boundaries
Addresses are specified as seen by the process specified by
.IR pid .
.PP
.I nodes
is an array of integers that specify the desired location for each page.
Each element in the array is a node number.
.I nodes
can also be NULL, in which case
.BR move_pages ()
does not move any pages but instead will return the node
where each page currently resides, in the
.I status
array.
Obtaining the status of each page may be necessary to determine
pages that need to be moved.
.PP
.I status
is an array of integers that return the status of each page.
The array contains valid values only if
.BR move_pages ()
did not return an error.
Preinitialization of the array to a value
which cannot represent a real numa node or valid error of status array
could help to identify pages that have been migrated.
.PP
.I flags
specify what types of pages to move.
.B MPOL_MF_MOVE
means that only pages that are in exclusive use by the process
are to be moved.
.B MPOL_MF_MOVE_ALL
means that pages shared between multiple processes can also be moved.
The process must be privileged
.RB ( CAP_SYS_NICE )
to use
.BR MPOL_MF_MOVE_ALL .
.SS Page states in the status array
The following values can be returned in each element of the
.I status
array.
.TP
.B 0..MAX_NUMNODES
Identifies the node on which the page resides.
.TP
.B \-EACCES
The page is mapped by multiple processes and can be moved only if
.B MPOL_MF_MOVE_ALL
is specified.
.TP
.B \-EBUSY
The page is currently busy and cannot be moved.
Try again later.
This occurs if a page is undergoing I/O or another kernel subsystem
is holding a reference to the page.
.TP
.B \-EFAULT
This is a zero page or the memory area is not mapped by the process.
.TP
.B \-EIO
Unable to write back a page.
The page has to be written back
in order to move it since the page is dirty and the filesystem
does not provide a migration function that would allow the move
of dirty pages.
.TP
.B \-EINVAL
A dirty page cannot be moved.
The filesystem does not
provide a migration function and has no ability to write back pages.
.TP
.B \-ENOENT
The page is not present.
.TP
.B \-ENOMEM
Unable to allocate memory on target node.
.SH RETURN VALUE
On success
.BR move_pages ()
returns zero.
.\" FIXME . Is the following quite true: does the wrapper in numactl
.\" do the right thing?
On error, it returns \-1, and sets
.I errno
to indicate the error.
If positive value is returned, it is the number of
nonmigrated pages.
.SH ERRORS
.TP
.B Positive value
The number of nonmigrated pages if they were the result of nonfatal
reasons (since
.\" commit a49bd4d7163707de377aee062f17befef6da891b
Linux 4.17).
.TP
.B E2BIG
Too many pages to move.
Since Linux 2.6.29,
.\" commit 3140a2273009c01c27d316f35ab76a37e105fdd8
the kernel no longer generates this error.
.TP
.B EACCES
.\" FIXME Clarify "current cpuset" in the description of the EACCES error.
.\" Is that the cpuset of the caller or the target?
One of the target nodes is not allowed by the current cpuset.
.TP
.B EFAULT
Parameter array could not be accessed.
.TP
.B EINVAL
Flags other than
.B MPOL_MF_MOVE
and
.B MPOL_MF_MOVE_ALL
was specified or an attempt was made to migrate pages of a kernel thread.
.TP
.B ENODEV
One of the target nodes is not online.
.TP
.B EPERM
The caller specified
.B MPOL_MF_MOVE_ALL
without sufficient privileges
.RB ( CAP_SYS_NICE ).
Or, the caller attempted to move pages of a process belonging
to another user but did not have privilege to do so
.RB ( CAP_SYS_NICE ).
.TP
.B ESRCH
Process does not exist.
.SH VERSIONS
.BR move_pages ()
first appeared on Linux in version 2.6.18.
.SH CONFORMING TO
This system call is Linux-specific.
.SH NOTES
Glibc does not provide a wrapper for this system call.
For information on library support, see
.BR numa (7).
.PP
Use
.BR get_mempolicy (2)
with the
.B MPOL_F_MEMS_ALLOWED
flag to obtain the set of nodes that are allowed by
.\" FIXME Clarify "current cpuset".  Is that the cpuset of the caller
.\" or the target?
the current cpuset.
Note that this information is subject to change at any
time by manual or automatic reconfiguration of the cpuset.
.PP
Use of this function may result in pages whose location
(node) violates the memory policy established for the
specified addresses (See
.BR mbind (2))
and/or the specified process (See
.BR set_mempolicy (2)).
That is, memory policy does not constrain the destination
nodes used by
.BR move_pages ().
.PP
The
.I <numaif.h>
header is not included with glibc, but requires installing
.I libnuma\-devel
or a similar package.
.SH SEE ALSO
.BR get_mempolicy (2),
.BR mbind (2),
.BR set_mempolicy (2),
.BR numa (3),
.BR numa_maps (5),
.BR cpuset (7),
.BR numa (7),
.BR migratepages (8),
.BR numastat (8)