# 页面分配算法

我们在`default_pmm.c`定义了一个pmm\_manager类型的结构体，并实现它的接口

```c
// kern/mm/default_pmm.h
#ifndef __KERN_MM_DEFAULT_PMM_H__
#define  __KERN_MM_DEFAULT_PMM_H__

#include <pmm.h>

extern const struct pmm_manager default_pmm_manager;

#endif /* ! __KERN_MM_DEFAULT_PMM_H__ */
```

较为关键的，是一开始如何初始化所有可用页面，以及如何分配和释放页面。这里实现了First Fit算法。

```c
// kern/mm/default_pmm.c
free_area_t free_area;

#define free_list (free_area.free_list)
#define nr_free (free_area.nr_free)

static void
default_init(void) {
    list_init(&free_list);
    nr_free = 0;//nr_free可以理解为在这里可以使用的一个全局变量，记录可用的物理页面数
}
static void
default_init_memmap(struct Page *base, size_t n) {
    assert(n > 0);
    struct Page *p = base;
    for (; p != base + n; p ++) {
        assert(PageReserved(p));
        p->flags = p->property = 0;
        set_page_ref(p, 0);
    }
    base->property = n;
    SetPageProperty(base);
    nr_free += n;
    if (list_empty(&free_list)) {
        list_add(&free_list, &(base->page_link));
    } else {
        list_entry_t* le = &free_list;
        while ((le = list_next(le)) != &free_list) {
            struct Page* page = le2page(le, page_link);
            if (base < page) {
                list_add_before(le, &(base->page_link));
                break;
            } else if (list_next(le) == &free_list) {
                list_add(le, &(base->page_link));
            }
        }
    }
}

static struct Page *
default_alloc_pages(size_t n) {
    assert(n > 0);
    if (n > nr_free) {
        return NULL;
    }
    struct Page *page = NULL;
    list_entry_t *le = &free_list;
    while ((le = list_next(le)) != &free_list) {
        struct Page *p = le2page(le, page_link);
        if (p->property >= n) {
            page = p;
            break;
        }
    }
    if (page != NULL) {
        list_entry_t* prev = list_prev(&(page->page_link));
        list_del(&(page->page_link));
        if (page->property > n) {
            struct Page *p = page + n;
            p->property = page->property - n;
            SetPageProperty(p);
            list_add(prev, &(p->page_link));
        }
        nr_free -= n;
        ClearPageProperty(page);
    }
    return page;
}
static void
default_free_pages(struct Page *base, size_t n) {
    assert(n > 0);
    struct Page *p = base;
    for (; p != base + n; p ++) {
        assert(!PageReserved(p) && !PageProperty(p));
        p->flags = 0;
        set_page_ref(p, 0);
    }
    base->property = n;
    SetPageProperty(base);
    nr_free += n;

    if (list_empty(&free_list)) {
        list_add(&free_list, &(base->page_link));
    } else {
        list_entry_t* le = &free_list;
        while ((le = list_next(le)) != &free_list) {
            struct Page* page = le2page(le, page_link);
            if (base < page) {
                list_add_before(le, &(base->page_link));
                break;
            } else if (list_next(le) == &free_list) {
                list_add(le, &(base->page_link));
            }
        }
    }

    list_entry_t* le = list_prev(&(base->page_link));
    if (le != &free_list) {
        p = le2page(le, page_link);
        if (p + p->property == base) {
            p->property += base->property;
            ClearPageProperty(base);
            list_del(&(base->page_link));
            base = p;
        }
    }

    le = list_next(&(base->page_link));
    if (le != &free_list) {
        p = le2page(le, page_link);
        if (base + base->property == p) {
            base->property += p->property;
            ClearPageProperty(p);
            list_del(&(p->page_link));
        }
    }
}

const struct pmm_manager default_pmm_manager = {
    .name = "default_pmm_manager",
    .init = default_init,
    .init_memmap = default_init_memmap,
    .alloc_pages = default_alloc_pages,
    .free_pages = default_free_pages,
    .nr_free_pages = default_nr_free_pages,
    .check = default_check,
};
```

目前为止的代码可以在[这里](https://github.com/Liurunda/riscv64-ucore/tree/lab2/lab2)找到，遇到困难可以参考。


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://1790865014.gitbook.io/ucore-step-by-step/intro-3/3_alloc.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.