Re: [PATCH v7 1/2] mm: introduce process_mrelease system call

From: Suren Baghdasaryan
Date: Thu Aug 05 2021 - 13:50:12 EST


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?

>
> --
> Thanks,
>
> David / dhildenb
>
> --
> To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@xxxxxxxxxxx.
>