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Subject[PATCH v5 7/7] mm/madvise: allow KSM hints for remote API
Date
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>
---
mm/madvise.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/mm/madvise.c b/mm/madvise.c
index 762a68205e65..0ecacfe93166 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -1004,6 +1004,10 @@ 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;
--
2.25.0.265.gbab2e86ba0-goog
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