On Tue, Jul 01, 2025 at 03:43:32PM -0600, Jonathan Corbet wrote:
Sasha Levin <sashal@xxxxxxxxxx> writes:
So I have a proof of concept which during the build process creates
.apispec.h which are generated from kerneldoc and contain macros
identical to the ones in my RFC.
Here's an example of sys_mlock() spec:
So I'm getting ahead of the game, but I have to ask some questions...
/**
* sys_mlock - Lock pages in memory
* @start: Starting address of memory range to lock
* @len: Length of memory range to lock in bytes
*
* 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.
*
* 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.
Why duplicate the long description?
Will fix.
* context-flags: KAPI_CTX_PROCESS | KAPI_CTX_SLEEPABLE
* param-type: start, KAPI_TYPE_UINT
This is something I wondered before; rather than a bunch of lengthy
KAPI_* symbols, why not just say __u64 (or some other familiar type)
here?
I think it gets tricky when we got to more complex types. For example,
how do we represent a FD or a (struct sockaddr *)?
With macros, KAPI_TYPE_FD or KAPI_TYPE_SOCKADDR make sense, but
__sockaddr will be a bit confusing (I think).
* param-flags: start, KAPI_PARAM_IN
* param-constraint-type: start, KAPI_CONSTRAINT_NONE
* param-constraint: start, Rounded down to page boundary
* param-type: len, KAPI_TYPE_UINT
* param-flags: len, KAPI_PARAM_IN
* param-constraint-type: len, KAPI_CONSTRAINT_RANGE
* param-range: len, 0, LONG_MAX
* param-constraint: len, Rounded up to page boundary
* return-type: KAPI_TYPE_INT
* return-check-type: KAPI_RETURN_ERROR_CHECK
* return-success: 0
* error-code: -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.
* error-code: -EPERM, EPERM, Insufficient privileges,
* The caller is not privileged (no CAP_IPC_LOCK) and RLIMIT_MEMLOCK is 0.
* error-code: -EINVAL, EINVAL, Address overflow,
* The result of the addition start+len was less than start (arithmetic overflow).
* error-code: -EAGAIN, EAGAIN, Some or all memory could not be locked,
* Some or all of the specified address range could not be locked.
* error-code: -EINTR, EINTR, Interrupted by signal,
* The operation was interrupted by a fatal signal before completion.
* error-code: -EFAULT, EFAULT, Bad address,
* The specified address range contains invalid addresses that cannot be accessed.
* since-version: 2.0
* lock: mmap_lock, KAPI_LOCK_RWLOCK
* lock-acquired: true
* lock-released: true
* lock-desc: Process memory map write lock
* signal: FATAL
* signal-direction: KAPI_SIGNAL_RECEIVE
* signal-action: KAPI_SIGNAL_ACTION_RETURN
* signal-condition: Fatal signal pending
* signal-desc: Fatal signals (SIGKILL) can interrupt the operation at two points:
* when acquiring mmap_write_lock_killable() and during page population
* in __mm_populate(). Returns -EINTR. Non-fatal signals do NOT interrupt
* mlock - the operation continues even if SIGINT/SIGTERM are received.
* signal-error: -EINTR
* signal-timing: KAPI_SIGNAL_TIME_DURING
* signal-priority: 0
* signal-interruptible: yes
* signal-state-req: KAPI_SIGNAL_STATE_RUNNING
* examples: mlock(addr, 4096); // Lock one page
* 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. Commonly used by real-time
* applications to prevent page faults during time-critical operations.
* Also used for security to prevent sensitive data (e.g., cryptographic keys)
* from being written to swap. Note: locked pages may still be saved to
* swap during system suspend/hibernate.
*
* Tagged addresses are automatically handled via untagged_addr(). The operation
* occurs in two phases: first VMAs are marked with VM_LOCKED, then pages are
* populated into memory. When checking RLIMIT_MEMLOCK, the kernel optimizes
* by recounting locked memory to avoid double-counting overlapping regions.
* side-effect: KAPI_EFFECT_MODIFY_STATE | KAPI_EFFECT_ALLOC_MEMORY, process memory, Locks pages into physical memory, preventing swapping, reversible=yes
I hope the really long lines starting here aren't the intended way to go...:)
I guess that we have two options around more complex blocks like these.
One, the longer lines you've pointed out. They are indeed long and
difficult to read, but they present a relatively static and "not too
interesting" information which users are likely to gloss over.
The other one would look something like:
side-effect: KAPI_EFFECT_MODIFY_STATE
side-effect-type: KAPI_EFFECT_MODIFY_STATE
side-effect-target: mm->locked_vm
side-effect-description: Increases process locked memory counter
side-effect-reversible: yes
Which isn't as long, but it occupies a bunch of vertical real estate
while not being too interesting for most of the readers.
* side-effect: KAPI_EFFECT_MODIFY_STATE, mm->locked_vm, Increases process locked memory counter, reversible=yes
* side-effect: KAPI_EFFECT_ALLOC_MEMORY, physical pages, May allocate and populate page table entries, condition=Pages not already present, reversible=yes
* side-effect: KAPI_EFFECT_MODIFY_STATE | KAPI_EFFECT_ALLOC_MEMORY, page faults, Triggers page faults to bring pages into memory, condition=Pages not already resident
* side-effect: KAPI_EFFECT_MODIFY_STATE, VMA splitting, May split existing VMAs at lock boundaries, condition=Lock range partially overlaps existing VMA
* state-trans: memory pages, swappable, locked in RAM, Pages become non-swappable and pinned in physical memory
* state-trans: VMA flags, unlocked, VM_LOCKED set, Virtual memory area marked as locked
* capability: CAP_IPC_LOCK, KAPI_CAP_BYPASS_CHECK, CAP_IPC_LOCK capability
* capability-allows: Lock unlimited amount of memory (no RLIMIT_MEMLOCK enforcement)
* capability-without: Must respect RLIMIT_MEMLOCK resource limit
* capability-condition: Checked when RLIMIT_MEMLOCK is 0 or locking would exceed limit
* capability-priority: 0
* constraint: RLIMIT_MEMLOCK Resource Limit, The RLIMIT_MEMLOCK soft resource limit specifies the maximum bytes of memory that may be locked into RAM. Unprivileged processes are restricted to this limit. CAP_IPC_LOCK capability allows bypassing this limit entirely. The limit is enforced per-process, not per-user.
* constraint-expr: RLIMIT_MEMLOCK Resource Limit, locked_memory + request_size <= RLIMIT_MEMLOCK || CAP_IPC_LOCK
* constraint: Memory Pressure and OOM, Locking large amounts of memory can cause system-wide memory pressure and potentially trigger the OOM killer. The kernel does not prevent locking memory that would destabilize the system.
* constraint: Special Memory Areas, Some memory types cannot be locked or are silently skipped: VM_IO/VM_PFNMAP areas (device mappings) are skipped; Hugetlb pages are inherently pinned and skipped; DAX mappings are always present in memory and skipped; Secret memory (memfd_secret) mappings are skipped; VM_DROPPABLE memory cannot be locked and is skipped; Gate VMA (kernel entry point) is skipped; VM_LOCKED areas are already locked. These special areas are silently excluded without error.
*
* Context: Process context. May sleep. Takes mmap_lock for write.
*
* Return: 0 on success, negative error code on failure
Both of these, of course, are much less informative versions of the data
you have put up above; it would be nice to unify them somehow.
Ack
--
Thanks,
Sasha
