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

[ron minnich <rminnich@gmail.com>]

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Right, so, getting back to the original discussion, Bakul, I think the
right path forward is to implement a device that supports an external
pager, rather than mmap.

But code wins, so, quick, somebody, implement something :-)

On Thu, Jan 8, 2026 at 11:03 PM David Leimbach via 9fans <9fans@9fans.net>
wrote:

> 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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