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    Subject[PATCH 4/4] mm/madvise: allow KSM hints for remote API
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    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@kernel.org>
    ---
    mm/madvise.c | 2 ++
    1 file changed, 2 insertions(+)

    diff --git a/mm/madvise.c b/mm/madvise.c
    index eb42b2b7f49b..3aa9aec6bfd9 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
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