set_mempolicy_home_node(2)

Purpose

set_mempolicy_home_node() sets the “home node” (the preferred NUMA node for the demotion/promotion balance path) on a VMA range whose policy is MPOL_BIND or MPOL_PREFERRED_MANY: for each VMA in [start, start + len) that already carries one of those two modes, the call duplicates the policy, assigns home_node, and rebinds the range (mm/mempolicy.c: mpol_dup + mbind_range). The home node acts as the target node when the kernel balances memory across the bound nodes (kernel commit history describes it as the node toward which hot pages are promoted).

Synopsis

/* kernel 5.17+, from mm/mempolicy.c (no man page, no glibc header) */
long set_mempolicy_home_node(unsigned long start, unsigned long len,
                             unsigned long home_node, unsigned long flags);

Arguments and behavior

start must be page-aligned (after untagged_addr) and len is page aligned by the kernel; a zero effective length returns 0 immediately (mm/mempolicy.c: if (end == start) return 0;). flags must be 0 (EINVAL otherwise). home_node must be below MAX_NUMNODES and online (!node_online(home_node) gives EINVAL). VMAs in the range without a policy are skipped; if any VMA in the range has a policy other than MPOL_BIND or MPOL_PREFERRED_MANY the call fails with EOPNOTSUPP and does not reset the home node for VMAs already updated (mm/mempolicy.c comment: “We don’t reset the home node for vmas we already updated before”).

Return value and errors

0 on success. Negated errors from the raw call: EINVAL (unaligned start, nonzero flags, offline or out-of-range home_node, overflow), ENOENT (no VMA in the range had a policy: err is initialized to -ENOENT), EOPNOTSUPP (a VMA in the range has a policy other than MPOL_BIND / MPOL_PREFERRED_MANY). The syscall takes mmap_write_lock for the operation.

Flags and constants

None: flags must be 0. Related constants MPOL_BIND (2) and MPOL_PREFERRED_MANY (5) live in the mbind table.

Security and notes

• No man page exists (man-pages 6.19); this page is a kernel-source entry, per the method’s source-limited convention – prose beyond the SYSCALL_DEFINE4 body (promotion/demotion interaction) is from mm/mempolicy.c context comments and is marked as such.
• i386, arm64-generic and arm32 share number 450 with x64 (hub).
• Worklog 1-b verified Since 5.17 by tag probing.

References

• Kernel source: source-cache/fetched/master-mm-mempolicy.c (SYSCALL_DEFINE4(set_mempolicy_home_node), lines ~1763-1830).
• Numbers/Since: hub overview; worklog Task 1-b.