Struct FrameMetaVtable 

#[repr(C)]
pub struct FrameMetaVtable {
    pub on_last_ref: Option<fn(PhysAddr)>,
    pub on_unmap: Option<fn(PhysAddr)>,
    pub reserved: [u64; 6],
}

Custom vtable for frame-type-specific behavior (DMA teardown, device hooks, …).

Store a pointer as u64 in MetaSlot::vtable; 0 selects DEFAULT_FRAME_META_VTABLE.

Fields

on_last_ref: Option<fn(PhysAddr)>

Called when the last shared reference to the frame is dropped (refcount → 0 path).

When it runs

Invoked by [release_owned_block] once for the head frame of a block, immediately after the last ownership reference is dropped and before any per-page on_unmap hooks. It does not run for individual page unmappings that leave the block pinned (e.g. one task unmapping while another still holds a pin).

Constraints

Invoked with IRQs disabled and without the buddy zone lock held. MUST be: allocation-free, lock-free, and infallible.

on_unmap: Option<fn(PhysAddr)>

Called once per 4 KiB page when a mapping block is released to the allocator (unmap path).

When it runs

Invoked by [release_owned_block] before the buddy allocator decides whether to recycle or quarantine the block. It therefore runs even for poisoned frames that will be quarantined and never reused.

Constraints

Invoked with IRQs disabled and the buddy zone lock held on the caller’s CPU. MUST be:

• allocation-free (no heap, no buddy);
• lock-free (no spinlocks that might be held by the interrupted CPU);
• infallible (no panic, no unwrap).

reserved: [u64; 6]

Reserved for future hooks (keeps struct at 64 bytes for FRAME_META_SIZE).