Moving page table mapping into page fault handler

[redamith2]

I am trying to move the mapping into the page fault handler, but i would like to create a pub static variables for both mapper and frame_allocator where I can pass it along to the mapping in the page fault handler

# in lib.rs
use memory::BootInfoFrameAllocator;
pub static mut FRAME_ALLOCATOR : Option<BootInfoFrameAllocator<Iterator<Item = PhysFrame>>> = None;
pub static mut MAPPER: Option<&'static mut RecursivePageTable> = None;

On doing so I get the error

doesn't have a size known at compile-time for Option<BootInfoFrameAllocator<Iterator<Item = PhysFrame>>>

[bjorn3]

dyn Iterator<Item = PhysFrame> doesnt have a statically known size, as any type implementing Iterator<Item = PhysFrame> could be behind it.

[phil-opp]

That's a tricky issue that I didn't consider. My intuition would be to use BootFrameAllocator<impl Iterator<…>> since otherwise it is treated like a dynamic trait object dyn Iterator as @bjorn3 said. However, it turns out this doesn't work either because the Rust compiler can't (yet) infer the correct type when a static is initialized with None.

I opened rust-lang/rust#60367 in the Rust repository for this. Let's see what they say.

[andre-richter]

In the meantime, I think you would need to pack the Iterator as a trait object, e.g. putting it in a Box.

pub struct BootInfoFrameAllocator<I> where I: Iterator<Item = PhysFrame> {
    frames: Box<I>,
}

[phil-opp]

@andre-richter That should work. The problem is that we don't have heap allocation on the blog yet.

[andre-richter]

Ah, my bad 😅

[phil-opp]

Ah, my bad sweat_smile

Not at all. It is the best solution to this problem currently, independent of the state of the blog.

[phil-opp]

I opened #595 to change the implementation of BootInfoFrameAllocator. The new implementation is non generic, so it should be usable in a static without problems.

[phil-opp]

Investing further, it seems like we could solve the original problem with a named existential type in the future. However, the implementation of this feature is still in its early stages (some ICEs andlifetime problems, only placeholder syntax), so it's not a good fit for the blog right now.

[phil-opp]

@redamith2 Could you try the updated instructions at https://os.phil-opp.com/paging-implementation/#allocating-frames and see if it works?

[redamith2]

Thank you I after making those changes it worked.

[phil-opp]

Perfect, thanks for trying!