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Add basic description for frame allocator methods

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Philipp Oppermann
2015-11-14 20:03:03 +01:00
parent 8304439c82
commit 16078431db
2 changed files with 89 additions and 66 deletions
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111
posts/DRAFT-allocating-frames.md
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@@ -211,10 +211,10 @@ pub trait FrameAllocator {
This allows us to create another, more advanced frame allocator in the future. This allows us to create another, more advanced frame allocator in the future.
### The Allocator ### The Allocator
Now we can put everything together and create the frame allocator. It looks like this: Now we can put everything together and create the actual frame allocator. It looks like this:
```rust ```rust
use memory::Frame; use memory::{Frame, FrameAllocator};
use multiboot2::{MemoryAreaIter, MemoryArea}; use multiboot2::{MemoryAreaIter, MemoryArea};
pub struct AreaFrameAllocator { pub struct AreaFrameAllocator {
@@ -227,7 +227,67 @@ pub struct AreaFrameAllocator {
multiboot_end: Frame, multiboot_end: Frame,
} }
``` ```
The `next_free_frame` field is a simple counter that is increased every time we return a frame. The `current_area` field holds the memory area that contains `next_free_frame`. If `next_free_frame` leaves this area, we will look for the next one in `areas`. The `{kernel, multiboot}_{start, end}` fields are used to avoid returning already used fields. The `next_free_frame` field is a simple counter that is increased every time we return a frame. The `current_area` field holds the memory area that contains `next_free_frame`. If `next_free_frame` leaves this area, we will look for the next one in `areas`. When there are no areas left, all frames are used and `current_area` becomes `None`. The `{kernel, multiboot}_{start, end}` fields are used to avoid returning already used fields.
To implement the `FrameAllocator` trait, we need to implement the `allocate_frame` and the `deallocate_frame` methods. The former looks like this:
```rust
fn allocate_frame(&mut self) -> Option<Frame> {
if let Some(area) = self.current_area {
let frame = self.next_free_frame;
// the last frame of the current area
let current_area_last_frame = {
let address = area.base_addr + area.length - 1;
Frame::containing_address(address as usize)
};
if frame > current_area_last_frame {
// all frames of current area are used, switch to next area
self.choose_next_area();
} else if frame >= self.kernel_start && frame <= self.kernel_end {
// `frame` is used by the kernel
self.next_free_frame = Frame {
number: self.kernel_end.number + 1
};
} else if frame >= self.multiboot_start && frame <= self.multiboot_end {
// `frame` is used by the multiboot information structure
self.next_free_frame = Frame {
number: self.multiboot_end.number + 1
};
} else {
// frame is unused, increment `next_free_frame` and return it
self.next_free_frame.number += 1;
return Some(frame);
}
// `frame` was not valid, try it again with the updated `next_free_frame`
self.allocate_frame()
} else {
None // no free frames left
}
}
```
The `choose_next_area` method isn't part of the trait and thus goes into an `impl AreaFrameAllocator` block:
```rust
fn choose_next_area(&mut self) {
self.current_area = self.areas.clone().filter(|area| {
let address = area.base_addr + area.length - 1;
Frame::containing_address(address as usize) >= self.next_free_frame
}).min_by(|area| area.base_addr);
}
```
This function chooses the area with the minimal base address that still has free frames, i.e. `next_free_frame` is smaller than its last frame. Note that we need to clone the iterator because the order of areas in the memory map isn't specified.
We don't have a data structure to store free frames, so we can't implement `deallocate_frame` reasonably. Thus we use the `unimplemented` macro, which just panics when called:
```rust
fn deallocate_frame(&mut self, _frame: Frame) {
unimplemented!()
}
```
Now we only need a constructor function:
```rust ```rust
pub fn new(kernel_start: usize, kernel_end: usize, pub fn new(kernel_start: usize, kernel_end: usize,
@@ -247,50 +307,7 @@ pub fn new(kernel_start: usize, kernel_end: usize,
allocator allocator
} }
``` ```
Note that we call `choose_next_area` manually here because `allocate_frame` returns `None` as soon as `current_area` is `None`.
```rust
fn choose_next_area(&mut self) {
self.current_area = self.areas.clone().filter(|area| {
let address = area.base_addr + area.length - 1;
Frame::containing_address(address as usize) >= self.next_free_frame
}).min_by(|area| area.base_addr);
}
```
```rust
fn allocate_frame(&mut self) -> Option<Frame> {
match self.current_area {
None => None,
Some(area) => {
let frame = self.next_free_frame;
let current_area_last_frame = {
let address = area.base_addr + area.length - 1;
Frame::containing_address(address as usize)
};
if frame > current_area_last_frame {
self.choose_next_area()
} else if frame >= self.kernel_start &&
frame <= self.kernel_end
{
self.next_free_frame = Frame {
number: self.kernel_end.number + 1
}
} else if frame >= self.multiboot_start &&
frame <= self.multiboot_end
{
self.next_free_frame = Frame {
number: self.multiboot_end.number + 1
}
} else {
self.next_free_frame.number += 1;
return Some(frame);
}
self.allocate_frame()
}
}
}
```
## Remapping the Kernel Sections ## Remapping the Kernel Sections
We can use the ELF section tag to write a skeleton that remaps the kernel correctly: We can use the ELF section tag to write a skeleton that remaps the kernel correctly:

44
src/memory/area_frame_allocator.rs
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@@ -43,27 +43,33 @@ impl AreaFrameAllocator {
impl FrameAllocator for AreaFrameAllocator { impl FrameAllocator for AreaFrameAllocator {
fn allocate_frame(&mut self) -> Option<Frame> { fn allocate_frame(&mut self) -> Option<Frame> {
match self.current_area { if let Some(area) = self.current_area {
None => None, let frame = self.next_free_frame;
Some(area) => {
let frame = self.next_free_frame;
let current_area_last_frame = {
let address = area.base_addr + area.length - 1;
Frame::containing_address(address as usize)
};
if frame > current_area_last_frame { // the last frame of the current area
self.choose_next_area() let current_area_last_frame = {
} else if frame >= self.kernel_start && frame <= self.kernel_end { let address = area.base_addr + area.length - 1;
self.next_free_frame = Frame{ number: self.kernel_end.number + 1 } Frame::containing_address(address as usize)
} else if frame >= self.multiboot_start && frame <= self.multiboot_end { };
self.next_free_frame = Frame{ number: self.multiboot_end.number + 1 }
} else { if frame > current_area_last_frame {
self.next_free_frame.number += 1; // all frames of current area are used, switch to next area
return Some(frame); self.choose_next_area();
} } else if frame >= self.kernel_start && frame <= self.kernel_end {
self.allocate_frame() // `frame` is used by the kernel
self.next_free_frame = Frame{ number: self.kernel_end.number + 1 };
} else if frame >= self.multiboot_start && frame <= self.multiboot_end {
// `frame` is used by the multiboot information structure
self.next_free_frame = Frame{ number: self.multiboot_end.number + 1 };
} else {
// frame is unused, increment `next_free_frame` and return it
self.next_free_frame.number += 1;
return Some(frame);
} }
// `frame` was not valid, try it again with the updated `next_free_frame`
self.allocate_frame()
} else {
None // no free frames left
} }
} }