Clarify memory usage limiting in case of --metadata-ttl enabled

[mhnap]

I have another question about --metadata-ttl following #855. Now it's explicitly possible to set --metadata-ttl to indefinite value. As I understand it's the same as setting the max value manually. But indefinite is even mentioned in the documentation, making it an expected usage. So, I have a question is there some logic implemented for memory usage limiting while using --metadata-ttl=indefinite? I don't know how much memory can be used by metadata caching for big buckets, but it's quite possible that there could be billions of objects inside. I would be grateful to know whether some memory liming logic exists or it's expected to grow indefinitely.

Thanks for fixing and documenting cache features!

[monthonk]

Hey, currently the kernel can call forget to remove a reference for an inode in mountpoint and we will remove that inode from the heap once there is no reference left. This mechanism relies on the kernel to free up some memory, so there is no guarantee that your memory usage would not grow. However, we are considering to add a new memory limiting mechanism in the future and we will make sure to update our doc for metadata accordingly.