On Thu, Aug 5, 2021 at 10:50 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > On Thu, Aug 5, 2021 at 10:29 AM David Hildenbrand <david@xxxxxxxxxx> wrote: > > > > 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@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/ > > > > > > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx> > > > --- > > > 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(). > > Yes, that's what we do after checking task_will_free_mem(). > task_will_free_mem() requires us to hold task_lock and > find_lock_task_mm() achieves that ensuring that mm is still valid, but > it might return a task other than the original one. That's why we do > this dance with pinning the new task and unpinning the original one. > The same dance is performed in __oom_kill_process(). I was > contemplating adding a parameter to find_lock_task_mm() to request > this unpin/pin be done within that function but then decided to keep > it simple for now. > Did I address your question or did I misunderstand it? > One question I have is why mmget() and not mmgrab()? I see mmgrab() in oom_kill.c.
