[RFC 10/19] mm/mlock: add API specification for mlock

Sasha Levin
2025-06-14T13:49:10Z

Add kernel API specification for the mlock() system call.

Signed-off-by: Sasha Levin <sashal@kernel.org>
---
mm/mlock.c | 105 +++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 105 insertions(+)

diff --git a/mm/mlock.c b/mm/mlock.c
index 3cb72b579ffd3..a37102df54b01 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -25,6 +25,7 @@
#include <linux/memcontrol.h>
#include <linux/mm_inline.h>
#include <linux/secretmem.h>
+#include <linux/syscall_api_spec.h>

#include "internal.h"

@@ -658,6 +659,110 @@ static __must_check int do_mlock(unsigned long start, size_t len, vm_flags_t fla
return 0;
}

+
+DEFINE_KERNEL_API_SPEC(sys_mlock)
+ KAPI_DESCRIPTION("Lock pages in memory")
+ KAPI_LONG_DESC("Locks pages in the specified address range into RAM, "
+ "preventing them from being paged to swap. Requires "
+ "CAP_IPC_LOCK capability or RLIMIT_MEMLOCK resource limit.")
+ KAPI_CONTEXT(KAPI_CTX_PROCESS | KAPI_CTX_SLEEPABLE)
+
+ KAPI_PARAM(0, "start", "unsigned long", "Starting address of memory range to lock")
+ .type = KAPI_TYPE_UINT,
+ KAPI_PARAM_FLAGS(KAPI_PARAM_IN)
+ .constraint_type = KAPI_CONSTRAINT_NONE,
+ .constraints = "Rounded down to page boundary",
+ KAPI_PARAM_END
+ KAPI_PARAM(1, "len", "size_t", "Length of memory range to lock in bytes")
+ .type = KAPI_TYPE_UINT,
+ KAPI_PARAM_FLAGS(KAPI_PARAM_IN)
+ .constraint_type = KAPI_CONSTRAINT_RANGE,
+ KAPI_PARAM_RANGE(0, LONG_MAX)
+ .constraints = "Rounded up to page boundary",
+ KAPI_PARAM_END
+
+ .return_spec = {
+ .type_name = "long",
+ .type = KAPI_TYPE_INT,
+ .check_type = KAPI_RETURN_ERROR_CHECK,
+ .success_value = 0,
+ .description = "0 on success, negative error code on failure",
+ },
+
+ KAPI_ERROR(0, -ENOMEM, "ENOMEM", "Address range issue",
+ "Some of the specified range is not mapped, has unmapped gaps, "
+ "or the lock would cause the number of mapped regions to exceed the limit.")
+ KAPI_ERROR(1, -EPERM, "EPERM", "Insufficient privileges",
+ "The caller is not privileged (no CAP_IPC_LOCK) and RLIMIT_MEMLOCK is 0.")
+ KAPI_ERROR(2, -EINVAL, "EINVAL", "Address overflow",
+ "The result of the addition start+len was less than start (arithmetic overflow).")
+ KAPI_ERROR(3, -EAGAIN, "EAGAIN", "Some or all memory could not be locked",
+ "Some or all of the specified address range could not be locked.")
+ KAPI_ERROR(4, -EINTR, "EINTR", "Interrupted by signal",
+ "The operation was interrupted by a fatal signal before completion.")
+
+ .error_count = 5,
+ .param_count = 2,
+ .since_version = "2.0",
+
+ .locks[0] = {
+ .lock_name = "mmap_lock",
+ .lock_type = KAPI_LOCK_RWLOCK,
+ .acquired = true,
+ .released = true,
+ .description = "Process memory map write lock",
+ },
+ .lock_count = 1,
+
+ /* Signal specifications */
+ .signal_count = 1,
+
+ /* Fatal signals can interrupt mmap_write_lock_killable */
+ KAPI_SIGNAL(0, 0, "FATAL", KAPI_SIGNAL_RECEIVE, KAPI_SIGNAL_ACTION_RETURN)
+ KAPI_SIGNAL_CONDITION("Fatal signal pending")
+ KAPI_SIGNAL_DESC("Fatal signals (SIGKILL, etc.) can interrupt the operation "
+ "when acquiring mmap_write_lock_killable(), causing -EINTR return")
+ KAPI_SIGNAL_END
+
+ .examples = "mlock(addr, 4096); // Lock one page\n"
+ "mlock(addr, len); // Lock range of pages",
+ .notes = "Memory locks do not stack - multiple calls on the same range can be "
+ "undone by a single munlock. Locks are not inherited by child processes. "
+ "Pages are locked on whole page boundaries.",
+
+ /* Side effects */
+ KAPI_SIDE_EFFECT(0, KAPI_EFFECT_MODIFY_STATE | KAPI_EFFECT_ALLOC_MEMORY,
+ "process memory",
+ "Locks pages into physical memory, preventing swapping")
+ KAPI_EFFECT_REVERSIBLE
+ KAPI_SIDE_EFFECT_END
+
+ KAPI_SIDE_EFFECT(1, KAPI_EFFECT_MODIFY_STATE,
+ "mm->locked_vm",
+ "Increases process locked memory counter")
+ KAPI_EFFECT_REVERSIBLE
+ KAPI_SIDE_EFFECT_END
+
+ KAPI_SIDE_EFFECT(2, KAPI_EFFECT_ALLOC_MEMORY,
+ "physical pages",
+ "May allocate and populate page table entries")
+ KAPI_EFFECT_CONDITION("Pages not already present")
+ KAPI_SIDE_EFFECT_END
+
+ KAPI_SIDE_EFFECT_COUNT(3)
+
+ /* State transitions */
+ KAPI_STATE_TRANS(0, "memory pages", "swappable", "locked in RAM",
+ "Pages become non-swappable and pinned in physical memory")
+ KAPI_STATE_TRANS_END
+
+ KAPI_STATE_TRANS(1, "VMA flags", "unlocked", "VM_LOCKED set",
+ "Virtual memory area marked as locked")
+ KAPI_STATE_TRANS_END
+
+ KAPI_STATE_TRANS_COUNT(2)
+KAPI_END_SPEC;
+
SYSCALL_DEFINE2(mlock, unsigned long, start, size_t, len)
{
return do_mlock(start, len, VM_LOCKED);
--
2.39.5