In modern systems it's not unusual to have a system component monitoring memory conditions of the system and tasked with keeping system memory pressure under control. One way to accomplish that is to kill non-essential processes to free up memory for more important ones. Examples of this are Facebook's OOM killer daemon called oomd and Android's low memory killer daemon called lmkd. For such system component it's important to be able to free memory quickly and efficiently. Unfortunately the time process takes to free up its memory after receiving a SIGKILL might vary based on the state of the process (uninterruptible sleep), size and OPP level of the core the process is running. A mechanism to free resources of the target process in a more predictable way would improve system's ability to control its memory pressure. Introduce process_mrelease system call that releases memory of a dying process from the context of the caller. This way the memory is freed in a more controllable way with CPU affinity and priority of the caller. The workload of freeing the memory will also be charged to the caller. The operation is allowed only on a dying process. Previously I proposed a number of alternatives to accomplish this: - https://lore.kernel.org/patchwork/patch/1060407 extending pidfd_send_signal to allow memory reaping using oom_reaper thread; - https://lore.kernel.org/patchwork/patch/1338196 extending pidfd_send_signal to reap memory of the target process synchronously from the context of the caller; - https://lore.kernel.org/patchwork/patch/1344419/ to add MADV_DONTNEED support for process_madvise implementing synchronous memory reaping. The end of the last discussion culminated with suggestion to introduce a dedicated system call (https://lore.kernel.org/patchwork/patch/1344418/#1553875) The reasoning was that the new variant of process_madvise a) does not work on an address range b) is destructive c) doesn't share much code at all with the rest of process_madvise >From the userspace point of view it was awkward and inconvenient to provide memory range for this operation that operates on the entire address space. Using special flags or address values to specify the entire address space was too hacky. The API is as follows, int process_mrelease(int pidfd, unsigned int flags); DESCRIPTION The process_mrelease() system call is used to free the memory of a process which was sent a SIGKILL signal. The pidfd selects the process referred to by the PID file descriptor. (See pidofd_open(2) for further information) The flags argument is reserved for future use; currently, this argument must be specified as 0. RETURN VALUE On success, process_mrelease() returns 0. On error, -1 is returned and errno is set to indicate the error. ERRORS EBADF pidfd is not a valid PID file descriptor. EAGAIN Failed to release part of the address space. EINVAL flags is not 0. EINVAL The task does not have a pending SIGKILL or its memory is shared with another process with no pending SIGKILL. ENOSYS This system call is not supported by kernels built with no MMU support (CONFIG_MMU=n). ESRCH The target process does not exist (i.e., it has terminated and been waited on). Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx> --- mm/oom_kill.c | 55 +++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 55 insertions(+) diff --git a/mm/oom_kill.c b/mm/oom_kill.c index d04a13dc9fde..7fbfa70d4e97 100644 --- a/mm/oom_kill.c +++ b/mm/oom_kill.c @@ -28,6 +28,7 @@ #include <linux/sched/task.h> #include <linux/sched/debug.h> #include <linux/swap.h> +#include <linux/syscalls.h> #include <linux/timex.h> #include <linux/jiffies.h> #include <linux/cpuset.h> @@ -755,10 +756,64 @@ static int __init oom_init(void) return 0; } subsys_initcall(oom_init) + +SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags) +{ + struct pid *pid; + struct task_struct *task; + struct mm_struct *mm = NULL; + unsigned int f_flags; + long ret = 0; + + if (flags != 0) + return -EINVAL; + + pid = pidfd_get_pid(pidfd, &f_flags); + if (IS_ERR(pid)) + return PTR_ERR(pid); + + task = get_pid_task(pid, PIDTYPE_PID); + if (!task) { + ret = -ESRCH; + goto put_pid; + } + + /* + * If the task is dying and in the process of releasing its memory + * then get its mm. + */ + task_lock(task); + if (task_will_free_mem(task) && (task->flags & PF_KTHREAD) == 0) { + mm = task->mm; + mmget(mm); + } + task_unlock(task); + if (!mm) { + ret = -EINVAL; + goto put_task; + } + + mmap_read_lock(mm); + if (!__oom_reap_task_mm(mm)) + ret = -EAGAIN; + mmap_read_unlock(mm); + + mmput(mm); +put_task: + put_task_struct(task); +put_pid: + put_pid(pid); + return ret; +} #else static inline void wake_oom_reaper(struct task_struct *tsk) { } + +SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags) +{ + return -ENOSYS; +} #endif /* CONFIG_MMU */ /** -- 2.32.0.402.g57bb445576-goog
