linux-kernel - Re: [PATCH RFC v2 0/4] mm/ksm: add option to automerge VMAs

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Message-ID: <20190515062523.5ndf7obzfgugilfs@butterfly.localdomain>
Date:   Wed, 15 May 2019 08:25:23 +0200
From:   Oleksandr Natalenko <oleksandr@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Kirill Tkhai <ktkhai@...tuozzo.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Timofey Titovets <nefelim4ag@...il.com>,
        Aaron Tomlin <atomlin@...hat.com>,
        Grzegorz Halat <ghalat@...hat.com>, linux-mm@...ck.org,
        linux-api@...r.kernel.org, Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH RFC v2 0/4] mm/ksm: add option to automerge VMAs

Hi.

On Tue, May 14, 2019 at 04:51:22PM +0200, Michal Hocko wrote:
> [Forgot Hugh]
> 
> On Tue 14-05-19 16:41:05, Michal Hocko wrote:
> > [This is adding a new user visible interface so you should be CCing
> > linux-api mailing list. Also CC Hugh for KSM in general. Done now]

Right, thanks for taking care of this.

> > On Tue 14-05-19 15:16:50, Oleksandr Natalenko wrote:
> > > By default, KSM works only on memory that is marked by madvise(). And the
> > > only way to get around that is to either:
> > > 
> > >   * use LD_PRELOAD; or
> > >   * patch the kernel with something like UKSM or PKSM.
> > > 
> > > Instead, lets implement a sysfs knob, which allows marking VMAs as
> > > mergeable. This can be used manually on some task in question or by some
> > > small userspace helper daemon.
> > > 
> > > The knob is named "force_madvise", and it is write-only. It accepts a PID
> > > to act on. To mark the VMAs as mergeable, use:
> > > 
> > >    # echo PID > /sys/kernel/mm/ksm/force_madvise
> > > 
> > > To unmerge all the VMAs, use the same approach, prepending the PID with
> > > the "minus" sign:
> > > 
> > >    # echo -PID > /sys/kernel/mm/ksm/force_madvise
> > > 
> > > This patchset is based on earlier Timofey's submission [1], but it doesn't
> > > use dedicated kthread to walk through the list of tasks/VMAs. Instead,
> > > it is up to userspace to traverse all the tasks in /proc if needed.
> > > 
> > > The previous suggestion [2] was based on amending do_anonymous_page()
> > > handler to implement fully automatic mode, but this approach was
> > > incorrect due to improper locking and not desired due to excessive
> > > complexity.
> > > 
> > > The current approach just implements minimal interface and leaves the
> > > decision on how and when to act to userspace.
> > 
> > Please make sure to describe a usecase that warrants adding a new
> > interface we have to maintain for ever.

I think of two major consumers of this interface:

1) hosts, that run containers, especially similar ones and especially in
a trusted environment;

2) heavy applications, that can be run in multiple instances, not
limited to opensource ones like Firefox, but also those that cannot be
modified.

I'll add this justification to the cover letter once I send another
iteration of this submission if necessary.

Thank you.

> > 
> > > 
> > > Thanks.
> > > 
> > > [1] https://lore.kernel.org/patchwork/patch/1012142/
> > > [2] http://lkml.iu.edu/hypermail/linux/kernel/1905.1/02417.html
> > > 
> > > Oleksandr Natalenko (4):
> > >   mm/ksm: introduce ksm_enter() helper
> > >   mm/ksm: introduce ksm_leave() helper
> > >   mm/ksm: introduce force_madvise knob
> > >   mm/ksm: add force merging/unmerging documentation
> > > 
> > >  Documentation/admin-guide/mm/ksm.rst |  11 ++
> > >  mm/ksm.c                             | 160 +++++++++++++++++++++------
> > >  2 files changed, 137 insertions(+), 34 deletions(-)
> > > 
> > > -- 
> > > 2.21.0
> > > 
> > 
> > -- 
> > Michal Hocko
> > SUSE Labs
> 
> -- 
> Michal Hocko
> SUSE Labs

-- 
  Best regards,
    Oleksandr Natalenko (post-factum)
    Senior Software Maintenance Engineer