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From:   Daniel Colascione <dancol@google.com>
Date:   Mon, 13 Jan 2020 11:18:23 -0800
Message-ID: <CAKOZuevwbQvrFWqy5GOm4RXuGszKLBvRs9i-KbAi3nPcHhwvSw@mail.gmail.com>
Subject: Re: [PATCH 2/4] mm: introduce external memory hinting API
To:     Kirill Tkhai <ktkhai@virtuozzo.com>
Cc:     Minchan Kim <minchan@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>,
        Linux API <linux-api@vger.kernel.org>, oleksandr@redhat.com,
        Suren Baghdasaryan <surenb@google.com>,
        Tim Murray <timmurray@google.com>,
        Sandeep Patil <sspatil@google.com>,
        Sonny Rao <sonnyrao@google.com>,
        Brian Geffon <bgeffon@google.com>,
        Michal Hocko <mhocko@suse.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Shakeel Butt <shakeelb@google.com>,
        John Dias <joaodias@google.com>
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On Mon, Jan 13, 2020, 12:47 AM Kirill Tkhai <ktkhai@virtuozzo.com> wrote:
> > +SYSCALL_DEFINE5(process_madvise, int, pidfd, unsigned long, start,
> > +             size_t, len_in, int, behavior, unsigned long, flags)
>
> I don't like the interface. The fact we have pidfd does not mean,
> we have to use it for new syscalls always. A user may want to set
> madvise for specific pid from console and pass pid as argument.
> pidfd would be an overkill in this case.
> We usually call "kill -9 pid" from console. Why shouldn't process_madvise()
> allow this?

All new APIs should use pidfds: they're better than numeric PIDs in
every way. If a program wants to allow users to specify processes by
numeric PID, it can parse that numeric PID, open the corresponding
pidfd, and then use that pidfd with whatever system call it wants.
It's not necessary to support numeric PIDs at the system call level to
allow a console program to identify a process by numeric PID.