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

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Date:   Thu, 16 Jan 2020 15:59:52 -0800
From:   Minchan Kim <minchan@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
        linux-api@...r.kernel.org, oleksandr@...hat.com,
        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>, ktkhai@...tuozzo.com,
        christian.brauner@...ntu.com, sjpark@...zon.de,
        Minchan Kim <minchan@...nel.org>,
        Minchan Kim <minchan@...gle.com>
Subject: [PATCH v2 4/5] mm/madvise: allow KSM hints for remote API

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/

Signed-off-by: Oleksandr Natalenko <oleksandr@...hat.com>
Signed-off-by: Minchan Kim <minchan@...gle.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;
-- 
2.25.0.rc1.283.g88dfdc4193-goog