This was partially discussed in #8431, in which we added support to pkeyutl, through the new 3.0 -rawin argument, to support "oneshot" sign/verify operations (such as Ed25519/Ed448).

At the moment we allocate a buffer as big as the filesize (hoping it does not fail) and use BIO_read() to copy the file contents into memory, before running the oneshot operation.

It is desirable to use mmap() to avoid creating a buffer in the heap and copy the file contents into it, for performance and to better support large files.

Unfortunately, quoting @levitte ,

mmap-ed files would be ok, if mmap was implemented on all platforms... sadly, it is not, so we're not getting away from the problem of loading the whole file...

So we need to enable the mmap alternative only on supported platforms.

Good first issue

I am marking this as a good first issue, even if it is not necessarily meant for beginners.
Still it seems like a quite self-contained and bite-sized change, and could be a nice gateway item to start to familiarize with the project development process, and as good a starting point as any to dig into OpenSSL.