Linux伙伴算法释放

### Linux Buddy Allocator Release Mechanism

In the context of Linux's memory management, particularly focusing on the buddy allocator used within kernel space, releasing a block back to the system involves ensuring that adjacent blocks which were previously split can be merged if they are also free. This merging process helps maintain larger contiguous chunks available for future allocations and reduces fragmentation.

When freeing pages using `free_pages()` or similar functions, the core logic checks whether neighboring buddies (blocks of equal size next to each other) are already marked as free. If so, these neighbors combine into a single larger block[^1].

The algorithm recursively attempts this consolidation up through higher-order levels until no further merges are possible or necessary. Each level represents progressively bigger sizes by powers of two; hence an order-0 page is just one physical page while an order-n set contains \(2^n\) pages together[^3].

This approach ensures efficient reuse of freed areas without excessive fragmentation over time. However, it does require careful tracking via data structures like bitmaps where bits represent allocation status across all potential segments maintained by the allocator.

void __free_pages(struct page *page, unsigned int order)
{
    struct zone *zone = page_zone(page);
    unsigned long combined_idx;
    
    // ... initialization code ...

    do {
        combined_idx = idx | (idx ^ 1); /* Try combining with lower buddy */
        
        if (!buddy_is_movable(zone, combined_idx, order))
            break;

        clear_bit(combined_idx, zone->managed_map);

        if (--order >= 0 && !test_and_clear_bit(idx >> 1, zone->combined_map)) {
            idx >>= 1;
            continue;
        }
            
        list_add(&page->lru, &zone->free_area[order].free_list);
        zone->free_area[order].nr_free++;
        
    } while(order-- > 0);
}

--related questions--

1. How does the buddy allocator handle small versus large memory requests differently?
2. What mechanisms exist in Linux to prevent thrashing due to frequent splitting and joining operations in the buddy system?
3. Can you explain how virtual addresses map to physical frames under demand paging alongside the role played by the buddy allocator during such mappings?
4. In what scenarios might direct use of low-level allocators like those based on the buddy method become preferable over high-level ones provided by user-space libraries?