What is the downside of memory mapping in this scenario? Shouldn't the page table properly handle the case of doing a single sequential read over a range of pages? Accessing the contents of a file doesn't seem like something caching would matter for. Do you mean that reading of sequential pages will keep adding to the cache compared to reading from a single page? That seems like a similar thing as before where they will be the first things gone from the cache anyways.
Caching and paging almost always matter, even on things like this. The core problem is that the filesystem won't prefetch for you, and you will be waiting to page fault several times over the length of the file. Another problem of the size of the working set is that you will be seeing several slow calls to (essentially) malloc in the kernel to hold all of that data, while using a small, preallocated structure will give you none of that trouble.
