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I'm going to point out the security aspect, and that e.g., Windows used to enable it system-wide before getting taught by security experts otherwise. Details on KSM and security aspects can be found in that thread.
If I'm not mistaken the security aspect exists today. When KSM is enabled with madvise this is the same.
Yes, and we mostly only use it for virtual machines -- and to be precise, guest memory only -- where it has to be enabled explicitly on a well documented basis ...
Impossible for an admin to force it on other parts of the hypervisor process that might be more security sensitive. Or on other arbitrary applications, for now.
Long story short: one has to be very careful with that and only enable it for very carefully selected worklads. Letting a workload opt-in on a VMA level is most probably safer than an admin blindly turning this on for random processes ... >>
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[1] https://lore.kernel.org/all/20220517092701.1662641-1-xu.xin16@zte.com.cn/ [2] https://lore.kernel.org/all/20220609055658.703472-1-xu.xin16@zte.com.cn/
My understanding is that there were problems with the patch and how it exposed KSM. The other objection was the enable-all configuration option.
I don't remember all the discussions, but one concern was how to handle processes that deliberately want to disable it on some parts of memory.
Anyhow, I cc'ed the relevant parties already.