[pstch]

KSM is also not totally free, a good amount of CPU time has to be spent to search for merging opportunities (this amount scales logarithmically with the amount of pages to scan).

There is also a simple security implication, since you now have the possibility to over-allocate your physical memory, and suffer great consequences if many pages are unshared at the same time. Merged pages can be swapped out, but since they need to be scanned again when they are swapped in, BEFORE being merged, there is a great potential for memory pressure spikes in some configurations.