76 lines
3.7 KiB
Markdown
76 lines
3.7 KiB
Markdown
# The kernel heap
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The [frame allocator](frame-allocator.md) hands out fixed 4 KiB physical frames;
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the [VMM](paging.md) maps pages into virtual addresses. The **kernel heap** sits on
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top of both to provide what the rest of the kernel actually wants: `alloc(n)` /
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`free(p)` for arbitrary byte sizes. It's the first real consumer of `map()`, and
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the thing that unlocks dynamic data structures — lists, hash maps, driver state,
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eventually a process table.
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It's generic kernel code (`src/heap.zig`): the allocator logic is
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architecture-neutral, using `arch.mapPage` and the frame allocator underneath.
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## A growable free-list allocator
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The algorithm is a classic **first-fit free list**:
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- The heap owns a virtual region. Free space is tracked as an **address-ordered
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singly linked list** of free blocks; each block begins with a 16-byte header
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(`size`, and a `next` link used while free).
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- **alloc(n)** walks the list for the first block big enough. If the block is much
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larger it's **split** — the front becomes the allocation, the remainder stays
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free. If nothing fits, the heap **grows** (below) and the search retries.
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- **free(p)** finds the block header just before `p` and inserts it back into the
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list, **coalescing** with the physically adjacent free blocks on either side so
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the space can be reused as one region rather than fragmenting away.
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Allocations are 16-byte aligned; larger alignments aren't supported yet (the
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`std.mem.Allocator` `alloc` returns `null` for them).
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## Growing on demand
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The heap lives in the **higher half** of the address space (virtual base
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`0xFFFF_8000_0000_0000`) — unmapped, well clear of the identity-mapped low half,
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and leaving the low half free for a future user address space. (That base is
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x86_64-canonical; another architecture would pick its own.)
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When the free list can't satisfy a request, `grow` extends the mapped region: it
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pulls fresh frames from the [frame allocator](frame-allocator.md) and `map`s each
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onto the end of the heap, then adds the new span as a free block (coalescing with
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the current tail). So the heap starts at one page and expands page-by-page as
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demand requires, up to a cap. This is exactly what the VMM's on-demand `map` was
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built for.
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## A std.mem.Allocator
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The heap is exposed as a **`std.mem.Allocator`** (`heap.allocator()`), Zig's
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standard allocator interface. That's a deliberate multiplier: it means the whole of
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Zig's standard library — `ArrayList`, `AutoHashMap`, `std.fmt.allocPrint`, and the
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rest — works directly on the kernel heap, no bespoke containers required.
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## Verifying it
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The `heap` test (see [testing.md](testing.md)) exercises the allocator end to end:
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```
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[PASS] alloc 4096 bytes
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[PASS] heap memory is writable and reads back
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[PASS] freed block is reused <- free list + coalescing works
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[PASS] many allocations (heap growth) stay valid <- grow() maps fresh frames
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[PASS] std.ArrayList on the kernel heap <- std containers work on it
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```
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The "freed block is reused" check (free then re-alloc returns the same address) is
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the proof that free and the free list actually work, not just alloc; "heap growth"
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forces allocation past the initial page so `grow`/`map` runs; and the `ArrayList`
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check is the std-integration payoff.
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## What's next (not done here)
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- **Thread/interrupt safety.** The heap assumes a single caller — no lock yet.
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It's safe now (nothing allocates from interrupt handlers), but threads or an
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allocating IRQ handler will need a lock (or `cli` around the critical section).
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- **Larger alignments** than 16 (for page-aligned buffers, DMA regions).
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- **`resize`/`remap` in place**, so growing an `ArrayList` needn't always copy.
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- **Reclaiming empty tail pages** back to the frame allocator when the heap shrinks.
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