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

[arnold@skeeve.com]

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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> >

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