Re: [PATCH v2 4/5] mm/madvise: allow KSM hints for remote API

[Minchan Kim <minchan@kernel.org>]

On Fri, Jan 17, 2020 at 01:34:00PM +0100, Oleksandr Natalenko wrote:
> Hi.
> 
> On Fri, Jan 17, 2020 at 01:13:14PM +0300, Kirill Tkhai wrote:
> > On 17.01.2020 02:59, Minchan Kim wrote:
> > > From: Oleksandr Natalenko <oleksandr@redhat.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/
> > > 
> > > Signed-off-by: Oleksandr Natalenko <oleksandr@redhat.com>
> > > Signed-off-by: Minchan Kim <minchan@google.com>
> > > ---
> > >  mm/madvise.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > > 
> > > diff --git a/mm/madvise.c b/mm/madvise.c
> > > index 84cffd0900f1..89557998d287 100644
> > > --- a/mm/madvise.c
> > > +++ b/mm/madvise.c
> > > @@ -1000,6 +1000,8 @@ process_madvise_behavior_valid(int behavior)
> > >  	switch (behavior) {
> > >  	case MADV_COLD:
> > >  	case MADV_PAGEOUT:
> > > +	case MADV_MERGEABLE:
> > > +	case MADV_UNMERGEABLE:
> > >  		return true;
> > >  	default:
> > >  		return false;
> > 
> > Remote madvise on KSM parameters should be OK.
> > 
> > One thing is madvise_behavior_valid() places MADV_MERGEABLE/UNMERGEABLE
> > in #ifdef brackes, so -EINVAL is returned by madvise() syscall if KSM
> > is not enabled. Here we should follow the same way for symmetry.
> > 
> 
> Thanks for the suggestion.
> 
> Minchan, shall you adopt it directly, or I should send a separate patch?

I will handle it in next spin.