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SubjectRe: [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@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/
> >
> > Reviewed-by: SeongJae Park <sjpark@amazon.de>
> > Signed-off-by: Oleksandr Natalenko <oleksandr@redhat.com>
> > Signed-off-by: Minchan Kim <minchan@kernel.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

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