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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Mon, 9 Aug 2021 09:23:07 -0700
Message-ID: <CAJuCfpG+4QAaxiqeXZ7xwrgTM-tuOhs+or2pPmKOsfKZFESh2A@mail.gmail.com>
Subject: Re: [PATCH v8 1/2] mm: introduce process_mrelease system call
To:     Shakeel Butt <shakeelb@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>,
        Michal Hocko <mhocko@suse.com>,
        David Rientjes <rientjes@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Roman Gushchin <guro@fb.com>, Rik van Riel <riel@surriel.com>,
        Minchan Kim <minchan@kernel.org>,
        Christian Brauner <christian@brauner.io>,
        Christoph Hellwig <hch@infradead.org>,
        Oleg Nesterov <oleg@redhat.com>,
        David Hildenbrand <david@redhat.com>,
        Jann Horn <jannh@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Florian Weimer <fweimer@redhat.com>,
        Jan Engelhardt <jengelh@inai.de>,
        Tim Murray <timmurray@google.com>,
        Linux API <linux-api@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        kernel-team <kernel-team@android.com>
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On Mon, Aug 9, 2021 at 8:23 AM Shakeel Butt <shakeelb@google.com> wrote:
>
> On Sun, Aug 8, 2021 at 9:08 AM Suren Baghdasaryan <surenb@google.com> 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@google.com>
>
> Reviewed-by: Shakeel Butt <shakeelb@google.com>

Thanks folks! I'll make the change Christian suggested and will push
the next version carrying Acks along, including the one from David
which I forgot to carry in this one.