Do I need to keep a file open after calling mmap on it?

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No, at least not on Linux it’s fine to close the file.

The manual page clearly states:

On the other hand, closing the file descriptor does not unmap the region.

For portability, I also checked the POSIX manual, it says the same thing (although even more clearly):

The mmap() function adds an extra reference to the file associated with the file descriptor fildes which is not removed by a subsequent close() on that file descriptor. This reference is removed when there are no more mappings to the file.

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