Re: mmaping on plan9? (was Re: [9fans] venti /plan9port mmapped

["Bakul Shah via 9fans" <9fans@9fans.net>]

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On Feb 12, 2026, at 12:37 AM, Alyssa M via 9fans <9fans@9fans.net> wrote:
> 
> So what I'm suggesting is that instead of the programmer making an mmap call, they should make a single read call to read the entire file into the address space - as they did before. The new read implementation would do this, but as a memory mapped snapshot. This looks no different to the programmer from how reads have always worked, it just happens very quickly, because no I/O actually happens.
> The snapshot data is brought in by demand paging as it is touched, and pages may get dirtied.

The issue here is the read call is given a pre-allocated buffer. What you seem to want is the read call to return as soon as possible and pages get read in when accessed. For this to work, the kernel side read implementation has to free up (or set aside) these pages, invalidate pagetable entries & flush relevant TLB entries & later reverse this operation once a page is filled in.

It is better if you let the read call itself manage memory. This was explored in a 1994 paper: "The Alloc Stream Facility": https://www.eecg.toronto.edu/~stumm/Papers/Krieger-IEEEComputer94.pdf 
For example,

stream = Sopen(filename, flags, ...)
...
addr = SallocAt(stream, mode, &length, offset, whence)
// can access memory in [addr..addr+length)
Sfree(stream, addr)
...
Sclose(stream)

The idea is SallocAt() returns a ptr to read in data  for read mode (or space for write mode) for length amount. Actual length is returned in the length variable if the file is too short. Sfree() frees up space or pushes out the data to the underlying file. The alloc stream facility can in theory also use mmap under the hood.

One can think of mmap() as something like sallocAt() fpr page aligned data. An added benefit: the same underlying data page can be accessed from multiple address spaces in a similar way. Any attempt to overwrite would result in copy-on-write so that other clients see the old data snapshot. If you want to see any updates by other processes, you should use some higher level access protocol to do so safely.

Using mmap instead of read is likely less efficient but the idea is to factor out common IO patterns.





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