高端内存映射之kmap_atomic固定映射--Linux内存管理(二十一)
1 固定映射
1.1 数据结构
linux高端内存中的临时内存区为固定内存区的一部分, 对于固定内存在linux内核中有下面描述
| 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的作用是将IO和BIOS以及物理地址空间映射到在896M至1G的128M的地址空间内, 使得kernel能够访问该空间并进行相应的读写操作。
start_kernel()->setup_arch()->early_ioremap_init()
然后arm和arm64上early_ioremap_init又是early_ioremap_setup的前端
| 函数 | 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 | 体系结构无关 | 体系结构无关 |
/*
* 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函数则会先调用early_fixmap_init函数来填充fixmap. 参见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函数的定义在
| 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架构的定义如下所示, 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);
/*得到固定映射区的pmd
，此pmd为虚拟地址转换为物理地址的pmd*/
pmd = fixmap_pmd(FIXADDR_TOP);
/*将bm_pte页表设置为固定映射区开始地址的pmd的第一个页表；*/
pmd_populate_kernel(&init_mm, pmd, bm_pte);
pte_offset_fixmap = pte_offset_early_fixmap;
}
1.3 ioremap函数
对于ioremap的使用需要通过early_memremap和early_iounmap进行.
由于对应于ioremap的内存空间是有限的, 所以对于ioremap空间的使用遵照使用结束马上释放的原则. 这就是说early_memremap和early_iounmap必须配对使用并且访问结束必须马上执行unmap
2 临时内核映射
刚才描述的kmap函数不能用于中断处理程序, 因为它可能进入睡眠状态. 如果pkmap数组中没有空闲位置, 该函数会进入睡眠状态, 直至情形有所改善.
因此内核提供了一个备选的映射函数, 其执行是原子的, 逻辑上称为kmap_atomic. 该函数的一个主要优点是它比普通的kmap快速. 但它不能用于可能进入睡眠的代码. 因此, 它对于很快就需要一个临时页的简短代码，是非常理想的.
kmap_atomic的定义在IA-32, PPC, Sparc32上是特定于体系结构的, 但这3种实现只有非常细微的差别. 其原型是相同的.
2.1 kmap_atomic函数
// http://lxr.free-electrons.com/source/arch/arm/mm/highmem.c?v=4.7#L55
void *kmap_atomic(struct page *page)
page是一个指向高端内存页的管理结构的指针, 而早期的内核中, 增加了一个类型为enum km_type的type参数, 用于指定所需的映射类型
// 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)
而在新的内核中, 删除了这个标识, 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);
这个函数不会被阻塞, 因此可以用在中断上下文和起亚不能重新调度的地方. 它也禁止内核抢占, 这是有必要的, 因此映射对每个处理器都是唯一的(调度可能对哪个处理器执行哪个进程做变动).
2.2 kunmap_atomic函数
可以通过函数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)
这个函数也不会阻塞. 在很多体系结构中, 除非激活了内核抢占, 否则kunmap_atomic根本无事可做, 因为只有在下一个临时映射到来前上一个临时映射才有效. 因此, 内核完全可以”忘掉”kmap_atomic映射, kunmap_atomic也无需做什么实际的事情. 下一个原子映射将自动覆盖前一个映射.
