On Mon, 24 Jan 2005, Rob Landley wrote:
Interesting. I wonder how that works? (PAE on x86 only lets you have 64G.)
Thats only an limitation of the CPU support for PAE. As UML is using mmap() other limits apply and these limits is mainly set by the UML pagetable structures.
But an individual process running under UML can still only have 4 gigabytes.
Not even that as we do not have a 4GB/4GB split in UML.
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.)
In theory UML HIGHMEM could be made to support even more pages, but it's besides the point.
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...
Thinking of HIGHMEM does hurt by design as you no longer have the equivalence of pointers to userspace in kernel land and must explicitly ask to have the needed userspace areas mapped/unmapped as needed, giving you the required pointers..
Most of this is dealt with automatically in copy_to/from_user, but some parts of the kernel uses other means to access userspace data and thus need to be very careful.
Regards Henrik
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