最近在看《深入理解LINUX内核》的高端内存映射，这里笔记下代码的实现吧。
highmem.c
/*
* High memory handling common code and variables.
*
* (C) 1999 Andrea Arcangeli, SuSE GmbH, andrea@suse.de
* Gerhard Wichert, Siemens AG, Gerhard.Wichert@pdb.siemens.de
*
*
* Redesigned the x86 32-bit VM architecture to deal with
* 64-bit physical space. With current x86 CPUs this
* means up to 64 Gigabytes physical RAM.
*
* Rewrote high memory support to move the page cache into
* high memory. Implemented permanent (schedulable) kmaps
* based on Linus' idea.
*
* Copyright (C) 1999 Ingo Molnar <mingo@redhat.com>
*/
#include <linux/mm.h>
#include <linux/module.h>
#include <linux/swap.h>
#include <linux/bio.h>
#include <linux/pagemap.h>
#include <linux/mempool.h>
#include <linux/blkdev.h>
#include <linux/init.h>
#include <linux/hash.h>
#include <linux/highmem.h>
#include <asm/tlbflush.h>
static mempool_t *page_pool, *isa_page_pool;
static void *page_pool_alloc(int gfp_mask, void *data)
{
int gfp = gfp_mask | (int) (long) data;
return alloc_page(gfp);
}
static void page_pool_free(void *page, void *data)
{
__free_page(page);
}
/*
* Virtual_count is not a pure "count".
* 0 means that it is not mapped, and has not been mapped
* since a TLB flush - it is usable.
* 1 means that there are no users, but it has been mapped
* since the last TLB flush - so we can't use it.
* n means that there are (n-1) current users of it.
*/
#ifdef CONFIG_HIGHMEM
static int pkmap_count[LAST_PKMAP];
static unsigned int last_pkmap_nr;
static __cacheline_aligned_in_smp DEFINE_SPINLOCK(kmap_lock);
pte_t * pkmap_page_table;
static DECLARE_WAIT_QUEUE_HEAD(pkmap_map_wait);
/*
刷新映射表，将计数为1的全部干掉，通过unmap可以发现，该函数不会主动彻底
释放映射，只是会唤醒等待队列，然后等待任务呗唤醒，通过循环进入本函数
*/
static void flush_all_zero_pkmaps(void)
{
int i;
flush_cache_kmaps();
for (i = 0; i < LAST_PKMAP; i++) {
struct page *page;
/*
* zero means we don't have anything to do,
* >1 means that it is still in use. Only
* a count of 1 means that it is free but
* needs to be unmapped
*/
if (pkmap_count[i] != 1)
continue;
pkmap_count[i] = 0;
/* sanity check */
if (pte_none(pkmap_page_table[i]))
BUG();
/*
* Don't need an atomic fetch-and-clear op here;
* no-one has the page mapped, and cannot get at
* its virtual address (and hence PTE) without first
* getting the kmap_lock (which is held here).
* So no dangers, even with speculative execution.
*/
page = pte_page(pkmap_page_table[i]);
pte_clear(&pkmap_page_table[i]);
/* 释放内存映射 */
set_page_address(page, NULL);
}
flush_tlb_kernel_range(PKMAP_ADDR(0), PKMAP_ADDR(LAST_PKMAP));
}
/*
*/
static inline unsigned long map_new_virtual(struct page *page)
{
unsigned long vaddr;
int count;
start:
count = LAST_PKMAP;
/* Find an empty entry */
/*
首先寻找一个计数为0的线性地址空间。计数为0说明该地址没有被映射
*/
for (;;) {
/* last_pkmap_nr为static，如果累加到头了，就在从头开始查询 */
last_pkmap_nr = (last_pkmap_nr + 1) & LAST_PKMAP_MASK;
if (!last_pkmap_nr) {
flush_all_zero_pkmaps();
count = LAST_PKMAP;
}
/* 找到了，返回 */
if (!pkmap_count[last_pkmap_nr])
break; /* Found a usable entry */
/*到头了，重置，不用再sleep了*/
if (--count)
continue;
/*
* Sleep for somebody else to unmap their entries
*/
/*
线性地址的永久映射的数目是有限的，如果找不到，只能sleep，
等待别人释放后再用
*/
{
DECLARE_WAITQUEUE(wait, current);
__set_current_state(TASK_UNINTERRUPTIBLE);
add_wait_queue(&pkmap_map_wait, &wait);
spin_unlock(&kmap_lock);
schedule();
remove_wait_queue(&pkmap_map_wait, &wait);
spin_lock(&kmap_lock);
/* Somebody else might have mapped it while we slept */
if (page_address(page))
return (unsigned long)page_address(page);
/* Re-start */
goto start;
}
}
/*如果有可用的线性地址，根据数组号反向查询对应的线性地址*/
vaddr = PKMAP_ADDR(last_pkmap_nr);
set_pte(&(pkmap_page_table[last_pkmap_nr]), mk_pte(page, kmap_prot));
/* 更新计数器 */
pkmap_count[last_pkmap_nr] = 1;
/* 将线性地址和对应的页描述符存到hash表 */
set_page_address(page, (void *)vaddr);
return vaddr;
}
/*
映射一个高端内存，传入为描述符地址，返回为线性地址
*/
void fastcall *kmap_high(struct page *page)
{
unsigned long vaddr;
/*
* For highmem pages, we can't trust "virtual" until
* after we have the lock.
*
* We cannot call this from interrupts, as it may block
*/
spin_lock(&kmap_lock);
/* 如果这个内存已经被映射，那么直接返回即可 */
vaddr = (unsigned long)page_address(page);
if (!vaddr)/* 之前没有映射，就映射 */
vaddr = map_new_virtual(page);
/*计数
根据ULK P308，必须远大于1才正确
*/
pkmap_count[PKMAP_NR(vaddr)]++;
if (pkmap_count[PKMAP_NR(vaddr)] < 2)
BUG();
spin_unlock(&kmap_lock);
return (void*) vaddr;
}
EXPORT_SYMBOL(kmap_high);
/*
取消一个内存映射
*/
void fastcall kunmap_high(struct page *page)
{
unsigned long vaddr;
unsigned long nr;
int need_wakeup;
spin_lock(&kmap_lock);
vaddr = (unsigned long)page_address(page);
if (!vaddr)
BUG();
nr = PKMAP_NR(vaddr);
/*
注意这里并没有彻底释放一个内存映射，只是表明该内存映射已经无用了，
然后唤醒等待队列(如果有任务在等待)，否则直接退出。
*/
/*
* A count must never go down to zero
* without a TLB flush!
*/
need_wakeup = 0;
switch (--pkmap_count[nr]) {
case 0:
BUG();
case 1:
/*
* Avoid an unnecessary wake_up() function call.
* The common case is pkmap_count[] == 1, but
* no waiters.
* The tasks queued in the wait-queue are guarded
* by both the lock in the wait-queue-head and by
* the kmap_lock. As the kmap_lock is held here,
* no need for the wait-queue-head's lock. Simply
* test if the queue is empty.
*/
need_wakeup = waitqueue_active(&pkmap_map_wait);
}
spin_unlock(&kmap_lock);
/* do wake-up, if needed, race-free outside of the spin lock */
if (need_wakeup)
wake_up(&pkmap_map_wait);
}
EXPORT_SYMBOL(kunmap_high);
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#if defined(HASHED_PAGE_VIRTUAL)
#define PA_HASH_ORDER 7
/*
* Describes one page->virtual association
*/
/*
高端地址映射时的结构体，存储在hash链表中，便于快速查找
*/
struct page_address_map {
struct page *page;/* 页表指针 */
void *virtual; /* 线性地址 */
struct list_head list;/* hash使用开链法存储，list为链表用*/
};
/*
* page_address_map freelist, allocated from page_address_maps.
*/
static struct list_head page_address_pool; /* freelist */
static spinlock_t pool_lock; /* protects page_address_pool */
/*
* Hash table bucket
*/
/*
定义一个hash表，用来存储不同的page指针(页描述符地址)对应的线性地址。
这里主要是管理高端内存的，因为高端内存无法直接映射一个线性地址，
必须通过内核映射才可以。
链表法存储hash元素:由于hash表的特性，不同的page可能会有相同的hash，因此hash表的结构体中
有一个锁，用来对这个结构体进行加锁，有一个链表，这个链表连接了
不同的page指针对应的page_address_map结构体。
*/
static struct page_address_slot {
struct list_head lh; /* List of page_address_maps */
spinlock_t lock; /* Protect this bucket's list */
} ____cacheline_aligned_in_smp page_address_htable[1<<PA_HASH_ORDER];
/*
工具函数，用来根据page指针获取对应的hash入口
*/
static struct page_address_slot *page_slot(struct page *page)
{
return &page_address_htable[hash_ptr(page, PA_HASH_ORDER)];
}
/*根据page指针(页描述符地址)获取对应的线性地址*/
void *page_address(struct page *page)
{
unsigned long flags;
void *ret;
struct page_address_slot *pas;
/*
如果不是高端内存，是(NORMAL DMA)，那么可以直接获取
*/
if (!PageHighMem(page))
return lowmem_page_address(page);
/*
如果是高端内存，就要查询映射表了
*/
/* 首先根据page指针查询对应的hash头 */
pas = page_slot(page);
ret = NULL;
/* 锁住对应的hash头 */
spin_lock_irqsave(&pas->lock, flags);
/*如果hash链表不为空，说明有元素在该hash中，那么就进行遍历，
看下是否有对应的page
*/
if (!list_empty(&pas->lh)) {
struct page_address_map *pam;/* 链表链的其实是page_address_map指针 */
list_for_each_entry(pam, &pas->lh, list) {
if (pam->page == page) {/* 如果遍历到了，就返回对应的线性地址 */
ret = pam->virtual;
goto done;
}
}
}
done:
spin_unlock_irqrestore(&pas->lock, flags);
return ret;
}
EXPORT_SYMBOL(page_address);
/*
将线性地址和对应的页描述符指针存到hash表中
这里额外做了删除功能，如果线性地址为空，说明
从hash表中删除对应的条目
*/
void set_page_address(struct page *page, void *virtual)
{
unsigned long flags;
struct page_address_slot *pas;
struct page_address_map *pam;
BUG_ON(!PageHighMem(page));
/*
hash表中的page_address_map结构体并不是临时申请的，而是一开始就申请好
然后连接在page_address_pool中的，使用时从里面取一个，用完了在释放回去
*/
pas = page_slot(page);
if (virtual) { /* Add */
BUG_ON(list_empty(&page_address_pool));
spin_lock_irqsave(&pool_lock, flags);
pam = list_entry(page_address_pool.next,
struct page_address_map, list);
list_del(&pam->list);
spin_unlock_irqrestore(&pool_lock, flags);
pam->page = page;
pam->virtual = virtual;
spin_lock_irqsave(&pas->lock, flags);
list_add_tail(&pam->list, &pas->lh);
spin_unlock_irqrestore(&pas->lock, flags);
} else { /* Remove */
spin_lock_irqsave(&pas->lock, flags);
list_for_each_entry(pam, &pas->lh, list) {
if (pam->page == page) {
list_del(&pam->list);
spin_unlock_irqrestore(&pas->lock, flags);
spin_lock_irqsave(&pool_lock, flags);
list_add_tail(&pam->list, &page_address_pool);
spin_unlock_irqrestore(&pool_lock, flags);
goto done;
}
}
spin_unlock_irqrestore(&pas->lock, flags);
}
done:
return;
}
static struct page_address_map page_address_maps[LAST_PKMAP];
void __init page_address_init(void)
{
int i;
INIT_LIST_HEAD(&page_address_pool);
for (i = 0; i < ARRAY_SIZE(page_address_maps); i++)
list_add(&page_address_maps[i].list, &page_address_pool);
for (i = 0; i < ARRAY_SIZE(page_address_htable); i++) {
INIT_LIST_HEAD(&page_address_htable[i].lh);
spin_lock_init(&page_address_htable[i].lock);
}
spin_lock_init(&pool_lock);
}
#endif /* defined(CONFIG_HIGHMEM) && !defined(WANT_PAGE_VIRTUAL) */
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