[RFC 13/19] mm/mlock: add API specification for munlock

Sasha Levin
2025-06-14T13:49:13Z

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

Signed-off-by: Sasha Levin <sashal@kernel.org>
---
mm/mlock.c | 129 +++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 129 insertions(+)

diff --git a/mm/mlock.c b/mm/mlock.c
index 95ee707c5922f..ef691adc78ad7 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -929,6 +929,135 @@ SYSCALL_DEFINE3(mlock2, unsigned long, start, size_t, len, int, flags)
return do_mlock(start, len, vm_flags);
}

+
+DEFINE_KERNEL_API_SPEC(sys_munlock)
+ KAPI_DESCRIPTION("Unlock pages in memory")
+ KAPI_LONG_DESC("Unlocks pages in the specified address range, allowing them "
+ "to be paged out to swap if needed.")
+ KAPI_CONTEXT(KAPI_CTX_PROCESS | KAPI_CTX_SLEEPABLE)
+
+ /* Parameters */
+ KAPI_PARAM(0, "start", "unsigned long", "Starting address of memory range to unlock")
+ KAPI_PARAM_FLAGS(KAPI_PARAM_IN)
+ .type = KAPI_TYPE_UINT,
+ .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 unlock in bytes")
+ KAPI_PARAM_FLAGS(KAPI_PARAM_IN)
+ .type = KAPI_TYPE_UINT,
+ .constraint_type = KAPI_CONSTRAINT_RANGE,
+ KAPI_PARAM_RANGE(0, LONG_MAX)
+ .constraints = "Rounded up to page boundary",
+ KAPI_PARAM_END
+
+ /* Return specification */
+ KAPI_RETURN("long", "0 on success, negative error code on failure")
+ .type = KAPI_TYPE_INT,
+ .check_type = KAPI_RETURN_ERROR_CHECK,
+ .success_value = 0,
+ KAPI_RETURN_END
+
+ /* Error codes */
+ KAPI_ERROR(0, -ENOMEM, "ENOMEM", "Memory range not mapped", "(Linux 2.6.9 and later) Some of the specified address range does not correspond to mapped pages in the process address space.")
+ KAPI_ERROR(1, -EINTR, "EINTR", "Interrupted by signal", "The operation was interrupted by a signal before completion.")
+ KAPI_ERROR(2, -EINVAL, "EINVAL", "Address overflow", "The result of the addition start+len was less than start (arithmetic overflow).")
+
+ /* Signal specifications */
+ KAPI_SIGNAL(0, 0, "FATAL_SIGNALS", KAPI_SIGNAL_RECEIVE, KAPI_SIGNAL_ACTION_RETURN)
+ KAPI_SIGNAL_CONDITION("Fatal signal pending during mmap_write_lock_killable")
+ KAPI_SIGNAL_DESC("Fatal signals (SIGKILL, SIGTERM, etc.) can interrupt the operation when acquiring mmap_write_lock_killable(), causing -EINTR return")
+ KAPI_SIGNAL_RESTARTABLE
+ KAPI_SIGNAL_END
+
+ /* Side effects */
+ KAPI_SIDE_EFFECT(0, KAPI_EFFECT_MODIFY_STATE,
+ "process memory",
+ "Unlocks pages, making them eligible for swapping")
+ KAPI_EFFECT_REVERSIBLE
+ KAPI_EFFECT_CONDITION("Pages were previously locked")
+ KAPI_SIDE_EFFECT_END
+
+ KAPI_SIDE_EFFECT(1, KAPI_EFFECT_MODIFY_STATE,
+ "mm->locked_vm",
+ "Decreases process locked memory counter")
+ KAPI_EFFECT_REVERSIBLE
+ KAPI_EFFECT_CONDITION("Pages were counted in locked_vm")
+ KAPI_SIDE_EFFECT_END
+
+ KAPI_SIDE_EFFECT(2, KAPI_EFFECT_MODIFY_STATE,
+ "VMA flags",
+ "Clears VM_LOCKED and VM_LOCKONFAULT from affected VMAs")
+ KAPI_EFFECT_REVERSIBLE
+ KAPI_SIDE_EFFECT_END
+
+ KAPI_SIDE_EFFECT(3, KAPI_EFFECT_MODIFY_STATE,
+ "page flags",
+ "Clears PG_mlocked flag from unlocked pages")
+ KAPI_EFFECT_CONDITION("Pages had PG_mlocked set")
+ KAPI_SIDE_EFFECT_END
+
+ KAPI_SIDE_EFFECT(4, KAPI_EFFECT_MODIFY_STATE,
+ "LRU lists",
+ "Moves pages from unevictable to appropriate LRU list")
+ KAPI_EFFECT_CONDITION("Pages were on unevictable list")
+ KAPI_SIDE_EFFECT_END
+
+ /* State transitions */
+ KAPI_STATE_TRANS(0, "memory pages",
+ "locked in RAM", "swappable",
+ "Pages become eligible for swap out")
+ KAPI_STATE_TRANS_END
+
+ KAPI_STATE_TRANS(1, "VMA flags",
+ "VM_LOCKED set", "VM_LOCKED cleared",
+ "Virtual memory areas no longer marked as locked")
+ KAPI_STATE_TRANS_END
+
+ KAPI_STATE_TRANS(2, "page residency",
+ "guaranteed resident", "may be swapped",
+ "Pages can now be evicted under memory pressure")
+ KAPI_STATE_TRANS_END
+
+ KAPI_STATE_TRANS(3, "process statistics",
+ "locked memory accounted", "normal memory accounting",
+ "Memory no longer counted against RLIMIT_MEMLOCK")
+ KAPI_STATE_TRANS_END
+
+ KAPI_STATE_TRANS(4, "page LRU status",
+ "unevictable list", "active/inactive list",
+ "Pages moved to normal LRU lists for reclaim")
+ KAPI_STATE_TRANS_COND("Pages were mlocked")
+ KAPI_STATE_TRANS_END
+
+ /* Locking information */
+ KAPI_LOCK(0, "mmap_lock", KAPI_LOCK_RWLOCK)
+ KAPI_LOCK_DESC("Process memory map write lock")
+ KAPI_LOCK_ACQUIRED
+ KAPI_LOCK_RELEASED
+ KAPI_LOCK_DESC("Protects VMA modifications during unlock operation")
+ KAPI_LOCK_END
+
+ KAPI_LOCK(1, "lru_lock", KAPI_LOCK_SPINLOCK)
+ KAPI_LOCK_DESC("Per-memcg LRU list lock")
+ KAPI_LOCK_ACQUIRED
+ KAPI_LOCK_RELEASED
+ KAPI_LOCK_DESC("Taken when moving pages from unevictable to normal LRU lists")
+ KAPI_LOCK_END
+
+ .error_count = 3,
+ .param_count = 2,
+ .since_version = "2.0",
+ .signal_count = 1,
+ .side_effect_count = 5,
+ .state_trans_count = 5,
+ .lock_count = 2,
+ .examples = "munlock(addr, 4096); // Unlock one page\n"
+ "munlock(addr, len); // Unlock range of pages",
+ .notes = "No special permissions required to unlock memory",
+KAPI_END_SPEC;
+
SYSCALL_DEFINE2(munlock, unsigned long, start, size_t, len)
{
int ret;
--
2.39.5