move_pages - move individual pages of a process to another node

#include <numaif.h>
long move_pages(int pid, unsigned long count, void *pages[.count],
 const int nodes[.count], int status[.count], int flags);

move_pages() moves the specified pages of the process pid to the memory nodes specified by nodes. The result of the move is reflected in status. The flags indicate constraints on the pages to be moved.

pid is the ID of the process in which pages are to be moved. If pid is 0, then move_pages() moves pages of the calling process.

To move pages in another process requires the following privileges:

• Up to and including Linux 4.12: the caller must be privileged (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.

• 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, permission is governed by a ptrace access mode PTRACE_MODE_READ_REALCREDS check with respect to the target process; see ptrace(2).

count is the number of pages to move. It defines the size of the three arrays pages, nodes, and status.

pages is an array of pointers to the pages that should be moved. These are pointers that should be aligned to page boundaries. Addresses are specified as seen by the process specified by pid.

nodes is an array of integers that specify the desired location for each page. Each element in the array is a node number. nodes can also be NULL, in which case move_pages() does not move any pages but instead will return the node where each page currently resides, in the status array. Obtaining the status of each page may be necessary to determine pages that need to be moved.

status is an array of integers that return the status of each page. The array contains valid values only if 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.

flags specify what types of pages to move. MPOL_MF_MOVE means that only pages that are in exclusive use by the process are to be moved. MPOL_MF_MOVE_ALL means that pages shared between multiple processes can also be moved. The process must be privileged (CAP_SYS_NICE) to use MPOL_MF_MOVE_ALL.

Page states in the status array

The following values can be returned in each element of the status array.

0..MAX_NUMNODES

Identifies the node on which the page resides.

-EACCES

The page is mapped by multiple processes and can be moved only if MPOL_MF_MOVE_ALL is specified.

-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.

-EFAULT

This is a zero page or the memory area is not mapped by the process.

-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.

-EINVAL

A dirty page cannot be moved. The filesystem does not provide a migration function and has no ability to write back pages.

-ENOENT

The page is not present.

-ENOMEM

Unable to allocate memory on target node.

On success move_pages() returns zero. On error, it returns -1, and sets errno to indicate the error. If positive value is returned, it is the number of nonmigrated pages.

get_mempolicy(2), mbind(2), set_mempolicy(2), numa(3), numa_maps(5), cpuset(7), numa(7), migratepages(8), numastat(8)