高端内存（临时内存映射）

先看相关的变量的意思：

enum km_type {
D(0)    KM_BOUNCE_READ,
D(1)    KM_SKB_SUNRPC_DATA,
D(2)    KM_SKB_DATA_SOFTIRQ,
D(3)    KM_USER0,
D(4)    KM_USER1,
D(5)    KM_BIO_SRC_IRQ,
D(6)    KM_BIO_DST_IRQ,
D(7)    KM_PTE0,
D(8)    KM_PTE1,
D(9)    KM_IRQ0,
D(10)   KM_IRQ1,
D(11)   KM_SOFTIRQ0,
D(12)   KM_SOFTIRQ1,
D(13)   KM_TYPE_NR
};

还有一个是：

enum fixed_addresses {
        FIX_HOLE,
        FIX_VDSO,
        FIX_DBGP_BASE,
        FIX_EARLYCON_MEM_BASE,
#ifdef CONFIG_X86_LOCAL_APIC
        FIX_APIC_BASE,  /* local (CPU) APIC) -- required for SMP or not */
#endif
#ifdef CONFIG_X86_IO_APIC
        FIX_IO_APIC_BASE_0,
        FIX_IO_APIC_BASE_END = FIX_IO_APIC_BASE_0 + MAX_IO_APICS-1,
#endif
#ifdef CONFIG_X86_VISWS_APIC
        FIX_CO_CPU,     /* Cobalt timer */
        FIX_CO_APIC,    /* Cobalt APIC Redirection Table */
        FIX_LI_PCIA,    /* Lithium PCI Bridge A */
        FIX_LI_PCIB,    /* Lithium PCI Bridge B */
#endif
#ifdef CONFIG_X86_F00F_BUG
        FIX_F00F_IDT,   /* Virtual mapping for IDT */
#endif
#ifdef CONFIG_X86_CYCLONE_TIMER
        FIX_CYCLONE_TIMER, /*cyclone timer register*/
#endif
#ifdef CONFIG_HIGHMEM
        FIX_KMAP_BEGIN, /* reserved pte's for temporary kernel mappings */
        FIX_KMAP_END = FIX_KMAP_BEGIN+(KM_TYPE_NR*NR_CPUS)-1,
#endif
#ifdef CONFIG_ACPI
        FIX_ACPI_BEGIN,
        FIX_ACPI_END = FIX_ACPI_BEGIN + FIX_ACPI_PAGES - 1,
#endif
#ifdef CONFIG_PCI_MMCONFIG
        FIX_PCIE_MCFG,
#endif
#ifdef CONFIG_PARAVIRT
        FIX_PARAVIRT_BOOTMAP,
#endif
        __end_of_permanent_fixed_addresses,
#define NR_FIX_BTMAPS           64
#define FIX_BTMAPS_NESTING      4
        FIX_BTMAP_END =
                __end_of_permanent_fixed_addresses + 512 -
                        (__end_of_permanent_fixed_addresses & 511),
        FIX_BTMAP_BEGIN = FIX_BTMAP_END + NR_FIX_BTMAPS*FIX_BTMAPS_NESTING - 1,
        FIX_WP_TEST,
#ifdef CONFIG_PROVIDE_OHCI1394_DMA_INIT
        FIX_OHCI1394_BASE,
#endif
        __end_of_fixed_addresses
};

下面进入正题，临时映射的入口：

# define kmap_atomic(page, type)        __kmap_atomic(page, type)
/* 临时映射的入口 */
void *__kmap_atomic(struct page *page, enum km_type type)
{
        return kmap_atomic_prot(page, type, kmap_prot);
}

具体实现的代码（有些地方还不是非常清楚，以后弄清楚来再来补充）：

# define kmap_atomic_prot(page, type, prot)     __kmap_atomic_prot(page, type, prot)
/* 和kmap相比会快一点，应为不需要全局锁,并且不需要处理global tlb（并且kmap是不能睡眠的，这里type没什么用） */
void *__kmap_atomic_prot(struct page *page, enum km_type type, pgprot_t prot)
{
        /* 编译期间特别的线性地址 */
        enum fixed_addresses idx;
        unsigned long vaddr;
        /*
         * 原子映射是基于每个CPU的，因此在当前cpu上禁止抢占，直到unmap
         * 的时候才开启，这样就不会到时原子映射的重入，毕竟如果禁用抢占
         * 的话，调用者进程就很可能在同一cpu重入kmap_atomic_prot，然后就
         * 映射到同一虚拟地址。一次不允许睡眠。  
         */
        preempt_disable();
        pagefault_disable();
        /* 如果不是高端内存 */
        if (!PageHighMem(page))
                return page_address(page);
        
        /* 递增type，保证下面公式起作用 */
        debug_kmap_atomic_prot(type);

        /* 下面的两行用来保证N个CPU调用kmap_atomic不会将page映射到一个地址 */
        idx = type + KM_TYPE_NR*smp_processor_id();
        vaddr = __fix_to_virt(FIX_KMAP_BEGIN + idx);

        WARN_ON_ONCE(!pte_none(*(kmap_pte-idx)));
        set_pte(kmap_pte-idx, mk_pte(page, prot));
        arch_flush_lazy_mmu_mode();

        return (void *)vaddr;
}