[PATCH NOTFORMERGE 5/5] mm/madvise: allow KSM hints for remote API

[Oleksandr Natalenko]

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@xxxxxxxxxx>
---
 mm/madvise.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/madvise.c b/mm/madvise.c
index 84f899b1b6da..e8f9c49794a3 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -991,6 +991,8 @@ process_madvise_behavior_valid(int behavior)
 	switch (behavior) {
 	case MADV_COLD:
 	case MADV_PAGEOUT:
+	case MADV_MERGEABLE:
+	case MADV_UNMERGEABLE:
 		return true;
 
 	default:
-- 
2.22.0