linux-kernel - Re: [PATCH v3 5/5] mm: support both pid and pidfd for process

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Date:   Tue, 11 Feb 2020 13:11:49 -0800
From:   Minchan Kim <minchan@...nel.org>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Linux API <linux-api@...r.kernel.org>, oleksandr@...hat.com,
        Suren Baghdasaryan <surenb@...gle.com>,
        Tim Murray <timmurray@...gle.com>,
        Daniel Colascione <dancol@...gle.com>,
        Sandeep Patil <sspatil@...gle.com>,
        Sonny Rao <sonnyrao@...gle.com>,
        Brian Geffon <bgeffon@...gle.com>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        John Dias <joaodias@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH v3 5/5] mm: support both pid and pidfd for process_madvise

On Mon, Feb 10, 2020 at 03:12:52PM -0800, Alexander Duyck wrote:
> On Mon, Jan 27, 2020 at 4:17 PM Minchan Kim <minchan@...nel.org> wrote:
> >
> > There is a demand[1] to support pid as well pidfd for process_madvise
> > to reduce unncessary syscall to get pidfd if the user has control of
> > the targer process(ie, they could gaurantee the process is not gone
> > or pid is not reused. Or, it might be okay to give a hint to wrong
> > process).
> 
> It looks like you misspelled several items in here including
> "unnecessary", "target", and "guarantee".

Thanks, will fix it.

> 
> > This patch aims for supporting both options like waitid(2). So, the
> > syscall is currently,
> >
> >         int process_madvise(int which, pid_t pid, void *addr,
> >                 size_t length, int advise, unsigned long flag);
> >
> > @which is actually idtype_t for userspace libray and currently,
> > it supports P_PID and P_PIDFD.
> >
> > [1]  https://lore.kernel.org/linux-mm/9d849087-3359-c4ab-fbec-859e8186c509@virtuozzo.com/
> > Signed-off-by: Minchan Kim <minchan@...nel.org>
> > ---
> >  include/linux/pid.h      |  1 +
> >  include/linux/syscalls.h |  3 ++-
> >  kernel/exit.c            | 17 -----------------
> >  kernel/pid.c             | 17 +++++++++++++++++
> >  mm/madvise.c             | 34 ++++++++++++++++++++++------------
> >  5 files changed, 42 insertions(+), 30 deletions(-)
> >
> > diff --git a/include/linux/pid.h b/include/linux/pid.h
> > index 998ae7d24450..023d9c3a8edc 100644
> > --- a/include/linux/pid.h
> > +++ b/include/linux/pid.h
> > @@ -75,6 +75,7 @@ extern const struct file_operations pidfd_fops;
> >  struct file;
> >
> >  extern struct pid *pidfd_pid(const struct file *file);
> > +extern struct pid *pidfd_get_pid(unsigned int fd);
> >
> >  static inline struct pid *get_pid(struct pid *pid)
> >  {
> 
> So really this is two patches interleaved. You have the moving of the
> pidfd_get_pid function and the update of the syscall. Personally I
> would make the function move a separate patch and place it before you
> define the syscall and fold the syscall changes into your original
> patch.
> 
> Doing that you wouldn't have to worry about the syscall changing in
> behavior midway through a bisect. It would either be there or it
> wouldn't.

Will try it.
Thanks!