Re: [PATCH v2 2/3] mm: introduce process_mrelease system call

[David Hildenbrand]

On 18.07.21 23:41, Suren Baghdasaryan wrote:
> 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.

To me, this looks a lot cleaner. Although I do wonder why we need two 
separate mechanisms to achieve the end goal

1. send sigkill
2. process_mrelease

As 2. doesn't make sense without 1. it somehow feels like it would be 
optimal to achieve both steps in a single syscall. But I remember there 
were discussions around that.

> 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;

Nit: reverse Christmas tree.

> +
> +	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);
> +	}

AFAIU, while holding the task_lock, task->mm won't change and we cannot 
see a concurrent exit_mm()->mmput(). So the mm structure and the VMAs 
won't go away while holding the task_lock(). I do wonder if we need the 
mmget() at all here.

Also, I wonder if it would be worth dropping the task_lock() while 
reaping - to unblock anybody else wanting to lock the task. Getting a 
hold of the mm and locking the mmap_lock would be sufficient I guess.


In general, looks quite good to me.

--
Thanks,

David / dhildenb