Date:   Thu, 5 Aug 2021 10:49:57 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Michal Hocko <mhocko@...e.com>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>, Rik van Riel <riel@...riel.com>,
        Minchan Kim <minchan@...nel.org>,
        Christian Brauner <christian@...uner.io>,
        Christoph Hellwig <hch@...radead.org>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Florian Weimer <fweimer@...hat.com>,
        Jan Engelhardt <jengelh@...i.de>,
        Tim Murray <timmurray@...gle.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...roid.com>
Subject: Re: [PATCH v7 1/2] mm: introduce process_mrelease system call

On Thu, Aug 5, 2021 at 10:29 AM David Hildenbrand <david@...hat.com> 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@google.com/
> > [2] https://lore.kernel.org/linux-api/20201113173448.1863419-1-surenb@google.com/
> > [3] https://lore.kernel.org/linux-api/20201124053943.1684874-3-surenb@google.com/
> > [4] https://lore.kernel.org/linux-api/20201223075712.GA4719@lst.de/
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> > ---
> > 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
>
> --
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