Re: [PATCH v2 2/3] mm: introduce process_mrelease system call

[Suren Baghdasaryan <surenb@google.com>]

On Wed, Jul 21, 2021 at 1:02 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 18.07.21 23:41, 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
> > pidfd_send_signal to allow memory reaping using oom_reaper thread;
> > - https://lore.kernel.org/patchwork/patch/1338196 extending
> > 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.
>
> To me, this looks a lot cleaner. Although I do wonder why we need two
> separate mechanisms to achieve the end goal
>
> 1. send sigkill
> 2. process_mrelease
>
> As 2. doesn't make sense without 1. it somehow feels like it would be
> optimal to achieve both steps in a single syscall. But I remember there
> were discussions around that.

Yep, we recently discussed the approach in this thread:
https://lore.kernel.org/patchwork/patch/1450952/#1652452

>
> >
> > The end of the last discussion culminated with suggestion to introduce a
> > dedicated system call (https://lore.kernel.org/patchwork/patch/1344418/#1553875)
> > 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.
> >
> >            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.
> >
> >            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.
> >
> >            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@google.com>
> > ---
> >   mm/oom_kill.c | 55 +++++++++++++++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 55 insertions(+)
> >
> > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > index d04a13dc9fde..7fbfa70d4e97 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>
> > @@ -755,10 +756,64 @@ static int __init oom_init(void)
> >       return 0;
> >   }
> >   subsys_initcall(oom_init)
> > +
> > +SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
> > +{
> > +     struct pid *pid;
> > +     struct task_struct *task;
> > +     struct mm_struct *mm = NULL;
> > +     unsigned int f_flags;
> > +     long ret = 0;
>
> Nit: reverse Christmas tree.

Ack. Will reorder like this:

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);
> > +     if (task_will_free_mem(task) && (task->flags & PF_KTHREAD) == 0) {
> > +             mm = task->mm;
> > +             mmget(mm);
> > +     }
>
> AFAIU, while holding the task_lock, task->mm won't change and we cannot
> see a concurrent exit_mm()->mmput(). So the mm structure and the VMAs
> won't go away while holding the task_lock(). I do wonder if we need the
> mmget() at all here.
>
> Also, I wonder if it would be worth dropping the task_lock() while
> reaping - to unblock anybody else wanting to lock the task. Getting a
> hold of the mm and locking the mmap_lock would be sufficient I guess.

Let me take a closer look at the locking sequence here and will follow
up afterwards.
Thanks for the review!

>
>
> In general, looks quite good to me.
>
> --
> Thanks,
>
> David / dhildenb
>