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

[Suren Baghdasaryan]

On Thu, Aug 5, 2021 at 10:56 AM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:
>
> 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.

You are likely right here. The caller's context probably can't be
considered a "real user" when reaping the mm. However, we take an
mmap_lock shortly after, so not sure if in practice there is much
difference.
Michal, WDYT?