Mmap means many things to many people. Using mmap is most often not a matter of performance as much as it is a matter of flexibility: being able to mmap files is about as close as most operating systems get to exposing the underlying page table hardware, which lets applications that aren't happy with the OS-provided interface essentially design their own. The canonical example is databases, which would really prefer to be operating systems except for the whole having to write hardware drivers thing. 9vx is another example.
If all you want is to map a few read-only (or at least non-write-back) files, like you'd need to implement dynamically-loaded shared libraries, that's not too hard to add. Back when I did the first cut of linuxemu, I changed segattach to accept 'file!a.out' as a segment name. If you've got a lot of these, you might need to change the segment array to be larger or perhaps dynamically sized. The particular interface I chose felt clumsy so I never even suggested putting it into the kernel, but mostly you can reuse the code that demand-loads ordinary Plan 9 binaries. The shared-library kind of mmap is by far the simplest mmap use case. Even allowing mmap as a surrogate for malloc would probably push you pretty far toward needing to rewrite the memory system, since the mmap-based mallocs tend to use a large number of small mappings. True mmap support in the Plan 9 kernel would require rewriting the entire memory system to work in terms of pages instead of the coarser regions the kernel calls segments (text, data, bss, stack). This would be a lot of work with almost zero benefit. I'd rather let those applications not happy with what Plan 9 provides just not use Plan 9. There's no need to be all things to all people. Also, on Plan 9, shared memory means that the memory at the time of rfork is shared. If one proc then does a segattach, the other proc sharing its memory will not see the segattach. The same is true of segdetach. This is another reason that you can't just reuse the existing mechanisms to hack in a traditional full-featured mmap. To really get those right you might need (gasp) TLB shootdowns, which would be even more work. It's nice that Plan 9's memory system is simple--just a few segments, each of which is a run of pages. "True" mmap support would destroy that. Please don't. Russ
