Linux is borrowing unused memory for disk caching. This makes it look like you are low on memory, but you are not! Everything is fine!
Disk caching makes the system much faster and more responsive! There are no downsides, except for confusing newbies. It does not take memory away from applications in any way, ever!
If your applications want more memory, they just take back a chunk that the disk cache borrowed. Disk cache can always be given back to applications immediately! You are not low on ram!
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