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

["David Leimbach via 9fans" <9fans@9fans.net>]

I’d been impressed by L4.  It’s certainly been deployed pretty broadly.

And it has recursive pagers but … not sure how that’s used in practice.

And there are a bunch of variants.
Sent from my iPhone

> On Jan 8, 2026, at 9:23 PM, arnold@skeeve.com wrote:
> 
> I vaguely remember someone being quoted as saying
> 
>        Microkernels don't have to be small. They just have to
>        not do much.
> 
> :-)
> 
> ron minnich <rminnich@gmail.com> wrote:
> 
>> I would not tar the idea of external pagers with the Mach tarbrush. Mach
>> was pretty much inefficient at everything, including external pagers.
>> External pagers can work well, when implemented well.
>> 
>>> On Thu, Jan 8, 2026 at 8:41 PM Paul Lalonde <paul.a.lalonde@gmail.com>
>>> wrote:
>>> 
>>> Did the same on GPUs/Xeon Phi, including in the texture units.  Very
>>> useful mechanism for abstracting compute with random access characteristics.
>>> 
>>> Paul
>>> 
>>>> On Wed, Jan 7, 2026, 1:35 p.m. ron minnich <rminnich@gmail.com> wrote:
>>>> what we had planned for harvey was a good deal simpler: designate a part
>>>> of the address space as a "bounce fault to user" space area.
>>>> 
>>>> When a page fault in that area occurred, info about the fault was sent to
>>>> an fd (if  it was opened) or a note handler.
>>>> 
>>>> user could could handle the fault or punt, as it saw fit. The fixup was
>>>> that user mode had to get the data to satisfy the fault, then tell the
>>>> kernel what to do.
>>>> 
>>>> This is much like the 35-years-ago work we did on AIX, called
>>>> external pagers at the time; or the more recent umap work,
>>>> https://computing.llnl.gov/projects/umap, used fairly widely in HPC.
>>>> 
>>>> If you go this route, it's a bit less complex than what you are proposing.
>>>> 
>>>> On Wed, Jan 7, 2026 at 1:09 PM Bakul Shah via 9fans <9fans@9fans.net>
>>>> wrote:
>>>> 
>>>>> 
>>>>> 
>>>>>> 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.
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