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permalink · #1 (**permalink**) 02-18-2008, 07:44 AM

Making, drinking tea and reading an opus magnum from Jason Kim:
[Charset ISO-8859-1 unsupported, filtering to ASCII...]
>
> Reason why I am asking is that this would be a nice way for userland
> processes to create a region of memory - (a subspace, if you would ;-)
> that can be shared across processes. Instead of creating discrete pages
> to be shared (ala SYSV SHM), why not allow userland processes to create
> (and thus share) discrete sub-regions - which can independently have
> pages allocated/freed within that subspace?

that's not what submaps are made for.
also since you would require some sort of allocator to be written
anyway what is the difference wrt using shmems anyway?

> This would be most useful for creating a "growable" (and thus
> shrinkable) region of CONTIGUOUS virtual memory that can be shared
> across processes - (much like what currently happens in the kernel ?)

submaps are not for sharing virtual spaces.
read that comment again please.

also. what NEEDS a contiguos virtual space for later
split in smaller allocs. what is the application is
that you require this for?

> In contrast SYSV SHM doesn't support contiguous regions across shmget()
> calls, save the "accidental" shmat() of two or more shmid'd regions next
> to each other in the client process's address space.

what do you need it for?
why not use mmap() ?

> Is there an alternate way to add in this functionality besides using
> submaps?

why? what needs it?

> Michael Shalayeff wrote:
> > Making, drinking tea and reading an opus magnum from Jason W. Kim:
> > [Charset ISO-8859-1 unsupported, filtering to ASCII...]
> >
> >>Hi, all
> >>
> >>I'm looking through the uvm code in 3.6, and ran across a puzzling
> >>comment -
> >>
> >>near uvm_map.c:2108
> >>/*
> >> * uvm_map_submap: punch down part of a map into a submap
> >> *
> >> * => only the kernel_map is allowed to be submapped
> >> * => the purpose of submapping is to break up the locking granularity
> >> * of a larger map
> >>
> >>My question is, why is the creation of submaps limited to only the
> >>kernel? (Neither the tech archives nor cranor's thesis answers this
> >>question so ...)
> >>
> >>Secondly, what would it take to get the submap functionality working for
> >>userland? Are there any specific "gotcha"'s that I need to be aware of
> >>for getting the submap stuff working in userland?
> >
> >
> > the question is: why?
> >
> > cu
> >
>

--
paranoic mickey (my employers have changed but, the name has remained)

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Re: UVM - why no user process submaps? [OpenBSD/TECH]