On Sun, Aug 08, 2021 at 09:08:22AM -0700, 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@xxxxxxxxxx/ > [2] https://lore.kernel.org/linux-api/20201113173448.1863419-1-surenb@xxxxxxxxxx/ > [3] https://lore.kernel.org/linux-api/20201124053943.1684874-3-surenb@xxxxxxxxxx/ > [4] https://lore.kernel.org/linux-api/20201223075712.GA4719@xxxxxx/ > > Link: https://lore.kernel.org/r/20210808160823.3553954-1-surenb@xxxxxxxxxx > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx> > --- > 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); Technically, we really want PIDTYPE_TGID here. Currently, a pidfd can't be created for a thread that isn't a thread-group leader. And while we do make sure that when a pidfd is created the thread is a thread-group leader, i.e. has a PIDTYPE_TGID entry in its struct pid we might in the future not carry this restriction and will allow pidfds to refer to a single thread. When we do that we need to take a good look at all users carefully. So I'd prefer if this is changed to task = get_pid_task(pid, PIDTYPE_TGID); to clearly express that the assumption is that this is a thread-group leader. Otherwise, Acked-by: Christian Brauner <christian.brauner@xxxxxxxxxx>
