Re: [uml-devel] memory

[Rob Landley <rob@landley.net>]

On Sunday 23 January 2005 08:11 pm, Jeff Dike wrote:
> It'll allocate a temp file of the appropriate size.  If it's larger than
> what it can use for physical memory, the rest will be allocated as highmem.
>  If you leave 2-level pagetables on, then you're limited to 4G. If you turn
> on 3-level pagetables, then you get up to 128G.

Interesting.  I wonder how that works?  (PAE on x86 only lets you have 64G.)

> rob@landley.net said:
> > Not  unless it's unmapping and remapping multiple mmaps.
>
> It is.

But an individual process running under UML can still only have 4 gigabytes.

> > (There's
> > large file  support, but trying to mmap a 5 gig chunk out of a large
> > file can't work:
>
> It does it a page at a time.
>
> > what would that mean?  How could you generate an
> > offset into the last meg?
>
> The LFS interfaces take 64-bit offsets.

Yeah.  (Ouch.  But yeah, I can see that working...)

> > What would the pointer _be_?)
>
> What pointer?

The virtual or physical address used to access memory.  (I'm guessing 
userspace programs running under UML are limited to 4 gigs, and UML is using 
page indexes and is thus limited to 4 billion pages, not 4 billion bytes.)

> > The parent kernel's highmem support still doesn't provide more than 4
> > gigabytes per application, and the UML kernel is one application.
>
> The host kernel's highmem support is totally irrelevant.  There doesn't
> even need to be highmem support on the host.

Just large file support, and the ability to mmap up to 4 gigs of memory at a 
time (with a starting offset potentially above 4 gigabytes), and unmap it and 
map a different 4 gigs when you switch to the next process...

Makes my brain hurt just thinking about it, but that could be caffeine 
withdrawl...

> 				Jeff

Rob


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