Re: [PATCH v6 6/7] mm/madvise: employ mmget_still_valid for write lock

From: Suren Baghdasaryan <surenb@google.com>
To: Oleksandr Natalenko <oleksandr@redhat.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@vger.kernel.org, Tim Murray <timmurray@google.com>,
	Daniel Colascione <dancol@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>,
	Joel Fernandes <joel@joelfernandes.org>,
	sj38.park@gmail.com, alexander.h.duyck@linux.intel.com,
	Jann Horn <jannh@google.com>
Subject: Re: [PATCH v6 6/7] mm/madvise: employ mmget_still_valid for write lock
Date: Mon, 2 Mar 2020 08:32:44 -0800	[thread overview]
Message-ID: <CAJuCfpE++EUbmhmr6+iutFk5Nd3teXy5Xr0y735LP25ciNKKcQ@mail.gmail.com> (raw)
In-Reply-To: <20200302073332.gn7lvhxmmv5pupyq@butterfly.localdomain>

On Sun, Mar 1, 2020 at 11:33 PM Oleksandr Natalenko
<oleksandr@redhat.com> wrote:
>
> Hello.
>
> On Fri, Feb 28, 2020 at 03:19:55PM -0800, Suren Baghdasaryan wrote:
> > On Tue, Feb 18, 2020 at 5:44 PM Minchan Kim <minchan@kernel.org> wrote:
> > >
> > > From: Oleksandr Natalenko <oleksandr@redhat.com>
> > >
> > > Do the very same trick as we already do since 04f5866e41fb. KSM hints
> > > will require locking mmap_sem for write since they modify vm_flags, so
> > > for remote KSM hinting this additional check is needed.
> > >
> > > Signed-off-by: Oleksandr Natalenko <oleksandr@redhat.com>
> > > Signed-off-by: Minchan Kim <minchan@kernel.org>
> > > ---
> > >  mm/madvise.c | 3 +++
> > >  1 file changed, 3 insertions(+)
> > >
> > > diff --git a/mm/madvise.c b/mm/madvise.c
> > > index f6d9b9e66243..c55a18fe71f9 100644
> > > --- a/mm/madvise.c
> > > +++ b/mm/madvise.c
> > > @@ -1118,6 +1118,8 @@ int do_madvise(struct task_struct *target_task, struct mm_struct *mm,
> > >         if (write) {
> > >                 if (down_write_killable(&mm->mmap_sem))
> > >                         return -EINTR;
> > > +               if (current->mm != mm && !mmget_still_valid(mm))
> >
> > mmget_still_valid() seems pretty light-weight, so why not just use
> > that without checking that the mm belongs to the current process
> > first?
>
> I'd keep the checks separate to a) do not functionally change current->mm
> == mm case; b) clearly separate the intention to call
> mmget_still_valid() only for remote access (using mmget_still_valid()
> for current->mm == mm does not make any sense here, IMO, since there's
> no possibility of expecting a core dump at this point); c) ease the job for
> reviewer once mmget_still_valid() is scheduled to be removed (I hope it
> eventually goes away indeed).
>

Makes sense. Thanks!

> >
> > > +                       goto skip_mm;
> > >         } else {
> > >                 down_read(&mm->mmap_sem);
> > >         }
> > > @@ -1169,6 +1171,7 @@ int do_madvise(struct task_struct *target_task, struct mm_struct *mm,
> > >         }
> > >  out:
> > >         blk_finish_plug(&plug);
> > > +skip_mm:
> > >         if (write)
> > >                 up_write(&mm->mmap_sem);
> > >         else
> > > --
> > > 2.25.0.265.gbab2e86ba0-goog
> > >
> >
>
> --
>   Best regards,
>     Oleksandr Natalenko (post-factum)
>     Principal Software Maintenance Engineer
>

Reviewed-by: Suren Baghdasaryan <surenb@google.com>