Re: [PATCH v4 1/2] mm: introduce process_mrelease system call

[Suren Baghdasaryan]

On Tue, Aug 3, 2021 at 1:39 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Mon 02-08-21 15:14:30, 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
>
> Please use the msg-id based urls https://lore.kernel.org/lkml/20190411014353.113252-3-surenb@xxxxxxxxxx/

Will do. Thanks!

>
> > pidfd_send_signal to allow memory reaping using oom_reaper thread;
> > - https://lore.kernel.org/patchwork/patch/1338196 extending
>
> https://lore.kernel.org/linux-api/20201113173448.1863419-1-surenb@xxxxxxxxxx/
>
> > 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.
>
> https://lore.kernel.org/linux-api/20201124053943.1684874-3-surenb@xxxxxxxxxx/
>
> > The end of the last discussion culminated with suggestion to introduce a
> > dedicated system call (https://lore.kernel.org/patchwork/patch/1344418/#1553875)
>
> https://lore.kernel.org/linux-api/20201223075712.GA4719@xxxxxx/
>
> > 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.
>
> This is not really precise. The implementation will allow to use the
> syscall on any exiting or fatal signal received process. Not just those
> that have been SIGKILLed, right? For the purpose of the man page I would
> go with exiting process for the wording.

Ack.

>
> >           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.
> >
> >           EINTR  The call was interrupted by a signal; see signal(7).
> >
> >           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.
>
> again, wording here. I would go with
>             EINVAL The memory of the task cannot be released because the
>                    process is not exiting, the address space is shared
>                    with an alive process or there is a core dump is in
>                    progress..

Ack.

> >
> >           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>
> > ---
> > changes in v4:
> > - Replaced mmap_read_lock() with mmap_read_lock_killable(), per Michal Hocko
> > - Added EINTR error in the manual pages documentation
> >
> >  mm/oom_kill.c | 58 +++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 58 insertions(+)
> >
> > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > index c729a4c4a1ac..86727794b0a8 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 +1142,60 @@ 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;
> > +     unsigned int f_flags;
> > +     struct pid *pid;
> > +     long ret = 0;
> > +
> > +     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);
>
> Don't we need find_lock_task_mm here?

Yes, we do. Will fix in the next rev.

>
> > +     if (task_will_free_mem(task) && (task->flags & PF_KTHREAD) == 0) {
> > +             mm = task->mm;
> > +             mmget(mm);
> > +     }
> > +     task_unlock(task);
> > +     if (!mm) {
>
> Do we want to treat MMF_OOM_SKIP as a failure?

Yeah, I don't think we want to create additional contention if
oom-killer is already working on this mm. Should we return EBUSY in
this case? Other possible options is ESRCH, indicating that this
process is a goner, so don't bother. WDYT?

>
> > +             ret = -EINVAL;
> > +             goto put_task;
> > +     }
> > +
> > +     if (mmap_read_lock_killable(mm)) {
> > +             ret = -EINTR;
> > +             goto put_mm;
> > +     }
> > +     if (!__oom_reap_task_mm(mm))
> > +             ret = -EAGAIN;
> > +     mmap_read_unlock(mm);
> > +
> > +put_mm:
> > +     mmput(mm);
> > +put_task:
> > +     put_task_struct(task);
> > +put_pid:
> > +     put_pid(pid);
> > +     return ret;
> > +#else
> > +     return -ENOSYS;
> > +#endif /* CONFIG_MMU */
> > +}
> > --
> > 2.32.0.554.ge1b32706d8-goog
>

Thanks for the review!

> --
> Michal Hocko
> SUSE Labs