Re: [PATCH v8 1/2] mm: introduce process_mrelease system call

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/ (local)
[2] https://lore.kernel.org/linux-api/20201113173448.1863419-1-surenb@google.com/ (local)
[3] https://lore.kernel.org/linux-api/20201124053943.1684874-3-surenb@google.com/ (local)
[4] https://lore.kernel.org/linux-api/20201223075712.GA4719@lst.de/ (local)

Link: https://lore.kernel.org/r/20210808160823.3553954-1-surenb@google.com (local)
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
changes in v8:
- Replaced mmget with mmgrab, per Shakeel Butt
- Refactored the code to simplify and fix the task_lock release issue,
per Michal Hocko

 mm/oom_kill.c | 70 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 70 insertions(+)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index c729a4c4a1ac..f8acc26f7300 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 +1142,72 @@ 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;
+	bool reap = true;
+	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);