Re: [PATCH v7 1/2] mm: introduce process_mrelease system call

[David Hildenbrand <david@redhat.com>]

On 05.08.21 19:08, 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.
> 
> After previous discussions [1, 2, 3] the decision was made [4] to introduce
> a dedicated system call to cover this use case.
> 
> 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
>            an exiting process.
> 
>            The pidfd selects the process referred to by the PID file
>            descriptor.
>            (See pidfd_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.
> 
>            EINTR  The call was interrupted by a signal; see signal(7).
> 
>            EINVAL flags is not 0.
> 
>            EINVAL The memory of the task cannot be released because the
>                   process is not exiting, the address space is shared
>                   with another live process or there is a core dump in
>                   progress.
> 
>            ENOSYS This system call is not supported, for example, without
>                   MMU support built into Linux.
> 
>            ESRCH  The target process does not exist (i.e., it has terminated
>                   and been waited on).
> 
> [1] https://lore.kernel.org/lkml/20190411014353.113252-3-surenb@google.com/
> [2] https://lore.kernel.org/linux-api/20201113173448.1863419-1-surenb@google.com/
> [3] https://lore.kernel.org/linux-api/20201124053943.1684874-3-surenb@google.com/
> [4] https://lore.kernel.org/linux-api/20201223075712.GA4719@lst.de/
> 
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> ---
> changes in v7:
> - Fixed pidfd_open misspelling, per Andrew Morton
> - Fixed wrong task pinning after find_lock_task_mm() issue, per Michal Hocko
> - Moved MMF_OOM_SKIP check before task_will_free_mem(), per Michal Hocko
> 
>   mm/oom_kill.c | 73 +++++++++++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 73 insertions(+)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index c729a4c4a1ac..a4d917b43c73 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>
> @@ -1141,3 +1142,75 @@ void pagefault_out_of_memory(void)
>   	out_of_memory(&oc);
>   	mutex_unlock(&oom_lock);
>   }
> +
> +SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
> +{
> +#ifdef CONFIG_MMU
> +	struct mm_struct *mm = NULL;
> +	struct task_struct *task;
> +	struct task_struct *p;
> +	unsigned int f_flags;
> +	struct pid *pid;
> +	long ret = 0;
> +
> +	if (flags)
> +		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.
> +	 */
> +	p = find_lock_task_mm(task);
> +	if (!p) {
> +		ret = -ESRCH;
> +		goto put_pid;
> +	}
> +	if (task != p) {
> +		get_task_struct(p);


Wouldn't we want to obtain the mm from p ? I thought that was the whole 
exercise of going via find_lock_task_mm().

-- 
Thanks,

David / dhildenb