高端內存映射之kmap_atomic固定映射--Linux內存管理(二十一)

1 固定映射

1.1 數據結構

linux高端內存中的臨時內存區為固定內存區的一部分, 對於固定內存在linux內核中有下面描述

x86	arm	arm64
arch/x86/include/asm/fixmap.h?v=4.7, line 67	arch/arm/include/asm/fixmap.h?v=4.7, line 11	arch/arm64/include/asm/fixmap.h?v=4.7, line 36

/*
 * Here we define all the compile-time 'special' virtual
 * addresses. The point is to have a constant address at
 * compile time, but to set the physical address only
 * in the boot process.
 *
 * These 'compile-time allocated' memory buffers are
 * page-sized. Use set_fixmap(idx,phys) to associate
 * physical memory with fixmap indices.
 *
 */
enum fixed_addresses {
    FIX_HOLE,

    /*
     * Reserve a virtual window for the FDT that is 2 MB larger than the
     * maximum supported size, and put it at the top of the fixmap region.
     * The additional space ensures that any FDT that does not exceed
     * MAX_FDT_SIZE can be mapped regardless of whether it crosses any
     * 2 MB alignment boundaries.
     *
     * Keep this at the top so it remains 2 MB aligned.
     */
#define FIX_FDT_SIZE        (MAX_FDT_SIZE + SZ_2M)
    FIX_FDT_END,
    FIX_FDT = FIX_FDT_END + FIX_FDT_SIZE / PAGE_SIZE - 1,

    FIX_EARLYCON_MEM_BASE,
    FIX_TEXT_POKE0,
    __end_of_permanent_fixed_addresses,

    /*
     * Temporary boot-time mappings, used by early_ioremap(),
     * before ioremap() is functional.
     */
#define NR_FIX_BTMAPS       (SZ_256K / PAGE_SIZE)
#define FIX_BTMAPS_SLOTS    7
#define TOTAL_FIX_BTMAPS    (NR_FIX_BTMAPS * FIX_BTMAPS_SLOTS)

    FIX_BTMAP_END = __end_of_permanent_fixed_addresses,
    FIX_BTMAP_BEGIN = FIX_BTMAP_END + TOTAL_FIX_BTMAPS - 1,

    /*
     * Used for kernel page table creation, so unmapped memory may be used
     * for tables.
     */
    FIX_PTE,
    FIX_PMD,
    FIX_PUD,
    FIX_PGD,

    __end_of_fixed_addresses
};

1.2 固定映射

ioremap的作用是將IO和BIOS以及物理地址空間映射到在896M至1G的128M的地址空間內, 使得kernel能夠訪問該空間並進行相應的讀寫操作。

start_kernel()->setup_arch()->early_ioremap_init()

然后arm和arm64上early_ioremap_init又是early_ioremap_setup的前端

函數	x86	arm	arm64
early_ioremap_init	arch/x86/mm/ioremap.c?v=4.7, line 445	arch/arm/mm/ioremap.c?v=4.7, line 489	arch/arm64/mm/ioremap.c?v=4.7, line 110
early_ioremap_setup	mm/early_ioremap.c?v=4.7, line 67	體系結構無關	體系結構無關

/*
 * Must be called after early_fixmap_init
 */
void __init early_ioremap_init(void)
{
    early_ioremap_setup();
}

但是arm和arm64下的setup_arch函數則會先調用early_fixmap_init函數來填充fixmap. 參見arch/arm/kernel/setup.c?v=4.7, line 1058和arch/arm64/kernel/setup.c?v=4.7, line 229.

void __init setup_arch(char **cmdline_p)
{
    early_fixmap_init();
    early_ioremap_init();
}

early_fixmap_init函數的定義在

arm	arm64
arch/arm/mm/mmu.c?v=4.7, line 385	arch/arm64/mm/mmu.c?v=4.7, line 676

其中arm架構的定義如下所示, 在arch/arm/mm/mmu.c?v=4.7, line 385

void __init early_fixmap_init(void)
{
    pmd_t *pmd;

    /*
     * The early fixmap range spans multiple pmds, for which
     * we are not prepared:
     */
    BUILD_BUG_ON((__fix_to_virt(__end_of_early_ioremap_region) >> PMD_SHIFT)
             != FIXADDR_TOP >> PMD_SHIFT);

    /*得到固定映射區的pmd
    ，此pmd為虛擬地址轉換為物理地址的pmd*/
    pmd = fixmap_pmd(FIXADDR_TOP);
     /*將bm_pte頁表設置為固定映射區開始地址的pmd的第一個頁表；*/
    pmd_populate_kernel(&init_mm, pmd, bm_pte);

    pte_offset_fixmap = pte_offset_early_fixmap;
}

1.3 ioremap函數

對於ioremap的使用需要通過early_memremap和early_iounmap進行.

由於對應於ioremap的內存空間是有限的, 所以對於ioremap空間的使用遵照使用結束馬上釋放的原則. 這就是說early_memremap和early_iounmap必須配對使用並且訪問結束必須馬上執行unmap

2 臨時內核映射

剛才描述的kmap函數不能用於中斷處理程序, 因為它可能進入睡眠狀態. 如果pkmap數組中沒有空閑位置, 該函數會進入睡眠狀態, 直至情形有所改善.

因此內核提供了一個備選的映射函數, 其執行是原子的, 邏輯上稱為kmap_atomic. 該函數的一個主要優點是它比普通的kmap快速. 但它不能用於可能進入睡眠的代碼. 因此, 它對於很快就需要一個臨時頁的簡短代碼，是非常理想的.

kmap_atomic的定義在IA-32, PPC, Sparc32上是特定於體系結構的, 但這3種實現只有非常細微的差別. 其原型是相同的.

2.1 kmap_atomic函數

//  http://lxr.free-electrons.com/source/arch/arm/mm/highmem.c?v=4.7#L55
void *kmap_atomic(struct page *page)

page是一個指向高端內存頁的管理結構的指針, 而早期的內核中, 增加了一個類型為enum km_type的type參數, 用於指定所需的映射類型

//  http://lxr.free-electrons.com/source/arch/arm/mm/highmem.c?v=2.6.32#L39
void *kmap_atomic(struct page *page, enum km_type type)

而在新的內核中, 刪除了這個標識, 但是保留了km_type的最大值KM_TYPE_NR

void *kmap_atomic(struct page *page)
{
    unsigned int idx;
    unsigned long vaddr;
    void *kmap;
    int type;

    preempt_disable();
    pagefault_disable();
    if (!PageHighMem(page))
        return page_address(page);

#ifdef CONFIG_DEBUG_HIGHMEM
    /*
     * There is no cache coherency issue when non VIVT, so force the
     * dedicated kmap usage for better debugging purposes in that case.
     */
    if (!cache_is_vivt())
        kmap = NULL;
    else
#endif
        kmap = kmap_high_get(page);
    if (kmap)
        return kmap;

    type = kmap_atomic_idx_push();

    idx = FIX_KMAP_BEGIN + type + KM_TYPE_NR * smp_processor_id();
    vaddr = __fix_to_virt(idx);
#ifdef CONFIG_DEBUG_HIGHMEM
    /*
     * With debugging enabled, kunmap_atomic forces that entry to 0.
     * Make sure it was indeed properly unmapped.
     */
    BUG_ON(!pte_none(get_fixmap_pte(vaddr)));
#endif
    /*
     * When debugging is off, kunmap_atomic leaves the previous mapping
     * in place, so the contained TLB flush ensures the TLB is updated
     * with the new mapping.
     */
    set_fixmap_pte(idx, mk_pte(page, kmap_prot));

    return (void *)vaddr;
}
EXPORT_SYMBOL(kmap_atomic);

這個函數不會被阻塞, 因此可以用在中斷上下文和起亞不能重新調度的地方. 它也禁止內核搶占, 這是有必要的, 因此映射對每個處理器都是唯一的(調度可能對哪個處理器執行哪個進程做變動).

2.2 kunmap_atomic函數

可以通過函數kunmap_atomic取消映射

/*
 * Prevent people trying to call kunmap_atomic() as if it were kunmap()
 * kunmap_atomic() should get the return value of kmap_atomic, not the page.
 */
#define kunmap_atomic(addr)                     \
do {                                \
    BUILD_BUG_ON(__same_type((addr), struct page *));       \
    __kunmap_atomic(addr);                  \
} while (0)

這個函數也不會阻塞. 在很多體系結構中, 除非激活了內核搶占, 否則kunmap_atomic根本無事可做, 因為只有在下一個臨時映射到來前上一個臨時映射才有效. 因此, 內核完全可以”忘掉”kmap_atomic映射, kunmap_atomic也無需做什么實際的事情. 下一個原子映射將自動覆蓋前一個映射.