linux-kernel - Re: [PATCH v7 7/7] mm/madvise: allow KSM hints for remote API

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Date:   Fri, 6 Mar 2020 14:41:46 +0100
From:   Oleksandr Natalenko <oleksandr@...hat.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>, linux-api@...r.kernel.org,
        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>,
        Jann Horn <jannh@...gle.com>,
        alexander.h.duyck@...ux.intel.com, sj38.park@...il.com,
        SeongJae Park <sjpark@...zon.de>
Subject: Re: [PATCH v7 7/7] mm/madvise: allow KSM hints for remote API

On Fri, Mar 06, 2020 at 02:13:49PM +0100, Vlastimil Babka wrote:
> On 3/2/20 8:36 PM, Minchan Kim wrote:
> > From: Oleksandr Natalenko <oleksandr@...hat.com>
> > 
> > It all began with the fact that 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.
> > 
> > (i skip ptrace can of worms here intentionally)
> > 
> > To overcome this restriction, lets employ a new remote madvise API. This
> > can be used by some small userspace helper daemon that will do auto-KSM
> > job for us.
> > 
> > I think of two major consumers of remote KSM hints:
> > 
> >   * hosts, that run containers, especially similar ones and especially in
> >     a trusted environment, sharing the same runtime like Node.js;
> > 
> >   * heavy applications, that can be run in multiple instances, not
> >     limited to opensource ones like Firefox, but also those that cannot be
> >     modified since they are binary-only and, maybe, statically linked.
> > 
> > Speaking of statistics, more numbers can be found in the very first
> > submission, that is related to this one [1]. For my current setup with
> > two Firefox instances I get 100 to 200 MiB saved for the second instance
> > depending on the amount of tabs.
> > 
> > 1 FF instance with 15 tabs:
> > 
> >    $ echo "$(cat /sys/kernel/mm/ksm/pages_sharing) * 4 / 1024" | bc
> >    410
> > 
> > 2 FF instances, second one has 12 tabs (all the tabs are different):
> > 
> >    $ echo "$(cat /sys/kernel/mm/ksm/pages_sharing) * 4 / 1024" | bc
> >    592
> > 
> > At the very moment I do not have specific numbers for containerised
> > workload, but those should be comparable in case the containers share
> > similar/same runtime.
> > 
> > [1] https://lore.kernel.org/patchwork/patch/1012142/
> > 
> > Reviewed-by: SeongJae Park <sjpark@...zon.de>
> > Signed-off-by: Oleksandr Natalenko <oleksandr@...hat.com>
> > Signed-off-by: Minchan Kim <minchan@...nel.org>
> 
> This will lead to one process calling unmerge_ksm_pages() of another. There's a
> (signal_pending(current)) test there, should it check also the other task,
> analogically to task 3?

Do we care about current there then? Shall we just pass mm into unmerge_ksm_pages and check the signals of the target task only, be it current or something else?

> Then break_ksm() is fine as it is, as ksmd also calls it, right?

I think break_ksm() cares only about mmap_sem protection, so we should
be fine here.

> 
> > ---
> >  mm/madvise.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index e77c6c1fad34..f4fa962ee74d 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -1005,6 +1005,10 @@ process_madvise_behavior_valid(int behavior)
> >  	switch (behavior) {
> >  	case MADV_COLD:
> >  	case MADV_PAGEOUT:
> > +#ifdef CONFIG_KSM
> > +	case MADV_MERGEABLE:
> > +	case MADV_UNMERGEABLE:
> > +#endif
> >  		return true;
> >  	default:
> >  		return false;
> > 
> 

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