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

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

> On Jan 7, 2026, at 8:41 AM, ori@eigenstate.org wrote:
> 
> Quoth Bakul Shah via 9fans <9fans@9fans.net>:
>> I have this idea that will horrify most of you!
>> 
>> 1. Create an mmap device driver. You ask it to a new file handle which you use to communicate about memory mapping.
>> 2. If you want to mmap some file, you open it and write its file descriptor along with other parameters (file offset, base addr, size, mode, flags) to your mmap file handle.
>> 3. The mmap driver sets up necessary page table entries but doesn't actually fetch any data before returning from the write.
>> 4. It can asynchronously kick off io requests on your behalf and fixup page table entries as needed.
>> 5. Page faults in the mmapped area are serviced by making appropriate read/write calls.
>> 6. Flags can be used to indicate read-ahead or write-behind for typical serial access.
>> 7. Similarly msync, munmap etc. can be implemented.
>> 
>> In a sneaky way this avoids the need for adding any mmap specific syscalls! But the underlying work would be mostly similar in either case.
>> 
>> The main benefits of mmap are reduced initial latency , "pay as you go" cost structure and ease of use. It is certainly more expensive than reading/writing the same amount of data directly from a program.
>> 
>> No idea how horrible a hack is needed to implement such a thing or even if it is possible at all but I had to share this ;-)
> 
> To what end? The problems with mmap have little to do with adding a syscall;
> they're about how you do things like communicating I/O errors. Especially
> when flushing the cache.
> 
> Imagine the following setup -- I've imported 9p.io:
> 
>        9fs 9pio
> 
> and then I map a file from it:
> 
>        mapped = mmap("/n/9pio/plan9/lib/words", OWRITE);
> 
> Now, I want to write something into the file:
> 
>        *mapped = 1234;
> 
> The cached version of the page is dirty, so the OS will
> eventually need to flush it back with a 9p Twrite; Let's
> assume that before this happens, the network goes down.
> 
> How do you communicate the error with userspace?

This was just a brainwave but...

You have a (control) connection with the mmap device to
set up mmap so might as well use it to convey errors!
This device would be strictly local to where a program
runs.

I'd even consider allowing a separate process to mmap,
by making an address space a first class object. That'd
move more stuff out of the kernel and allow for more
interesting/esoteric uses.
------------------------------------------
9fans: 9fans
Permalink: https://9fans.topicbox.com/groups/9fans/Te8d7c6e48b5c075b-M6fb0ef830c8e525cb591fb48
Delivery options: https://9fans.topicbox.com/groups/9fans/subscription