在前面我们讲解了kmalloc申请连续物理内存的操作,以及原理和基础cache . 在内核中还有另外一个接口函数那就是vmalloc,申请一片连续的虚拟地址空间,但不保证物理空间连续,实际上我们会想到用户空间的malloc,malloc它是标准的glibc封装的一个函数,最终实现是通过系统调用brk和
mmap来实现,以后在分析它的实现过程. 它就是申请连续的虚拟空间,但是不保证物理内存的连续,当然用户程序也不怎么关心这个问题,只所以会关心物理内存的连续性一般是由于设备驱动的使用,或者DMA. 但是vmalloc申请效率比较低,还会造成TLB抖动. 一般内核里常用kmalloc. 除非特殊需求,比如要获取大块内存时,实例就是当ko模块加载到内核运行时,即需要vmalloc.
释放函数:vfree
参考内核 3.8.13
这里是说32位的处理器,即最大寻址4G虚拟空间,(当然现在已经64位比较普及了,后续补上吧)而虚拟地址到物理地址的转化往往需要硬件的支持才能提高效率,即MMU。
当然前提需要os先建立页表PT. 在linux内核,这4G空间并不是完全给用户空间使用在高端0xC0000000 (3G开始)留给内核空间使用(x86默认配置,默认0-16M(DMA),16M-896M(Normal),896M-1G(128M)作为高端内存分配区域),当然这个区域也是可是配置的.).
kmalloc函数返回的是虚拟地址(). kmalloc特殊之处在于它分配的内存是物理上连续的,这对于要进行DMA的设备十分重要. 而用vmalloc分配的内存只是连续,物理地址不一定连续,不能直接用于DMA。我们可以参考一个图:(它是arm 32架构的内核虚拟地址分配图)
下面我们就看看vmalloc函数:(mm/vmalloc.c)
-
/**
-
* vmalloc - allocate virtually contiguous memory
-
* @size: allocation size
-
* Allocate enough pages to cover @size from the page level
-
* allocator and map them into contiguous kernel virtual space.
-
*
-
* For tight control over page level allocator and protection flags
-
* use __vmalloc() instead.
-
*/
-
void *vmalloc(unsigned long size)
-
{
-
return __vmalloc_node_flags(size, -1, GFP_KERNEL | __GFP_HIGHMEM);
-
}
这里我们只用关注size即可,而vmalloc优先从高端内存分配,并且可以睡眠.
继续:
-
static inline void *__vmalloc_node_flags(unsigned long size,
-
int node, gfp_t flags)
-
{
-
return __vmalloc_node(size, 1, flags, PAGE_KERNEL,
-
node, __builtin_return_address(0));
-
}
重点看一下__vmalloc_node:
-
/**
-
* __vmalloc_node - allocate virtually contiguous memory
-
* @size: allocation size
-
* @align: desired alignment
-
* @gfp_mask: flags for the page level allocator
-
* @prot: protection mask for the allocated pages
-
* @node: node to use for allocation or -1
-
* @caller: caller's return address
-
*
-
* Allocate enough pages to cover @size from the page level
-
* allocator with @gfp_mask flags. Map them into contiguous
-
* kernel virtual space, using a pagetable protection of @prot.
-
*/
-
static void *__vmalloc_node(unsigned long size, unsigned long align,
-
gfp_t gfp_mask, pgprot_t prot,
-
int node, const void *caller)
-
{
-
return __vmalloc_node_range(size, align, VMALLOC_START, VMALLOC_END,
-
gfp_mask, prot, node, caller);
-
}
因为这里提到了VMALLOC_START和VMALLOC_END它们究竟是什么值呢?
这里看了arm32和mips32的(根据架构虚拟地址分配不同而不同,比如mips就比较特殊):
在arch/mips/include/asm/pgtable-32.h中
首先看mips虚拟地址分布图:
从这个图里我们知道用户空间为2G(0x0-0x7fff ffff),dma或者normal内存映射在kseg0(512M)/kseg1,而对于vmalloc申请的虚拟地址在kseg2中,当然还有其他一些特殊的映射比如io等.
-
#define VMALLOC_START MAP_BASE
-
-
#define PKMAP_BASE (0xfe000000UL)
-
-
#ifdef CONFIG_HIGHMEM
-
# define VMALLOC_END (PKMAP_BASE-2*PAGE_SIZE)
-
#else
-
# define VMALLOC_END (FIXADDR_START-2*PAGE_SIZE)
-
#endif
在arch/arm/include/asm/pgtable.h
-
/*
-
* Just any arbitrary offset to the start of the vmalloc VM area: the
-
* current 8MB value just means that there will be a 8MB "hole" after the
-
* physical memory until the kernel virtual memory starts. That means that
-
* any out-of-bounds memory accesses will hopefully be caught.
-
* The vmalloc() routines leaves a hole of 4kB between each vmalloced
-
* area for the same reason. ;)
-
*/
-
#define VMALLOC_OFFSET (8*1024*1024)
-
#define VMALLOC_START (((unsigned long)high_memory + VMALLOC_OFFSET) & ~(VMALLOC_OFFSET-1))
-
#define VMALLOC_END 0xff000000UL
在看一个图:
我们知道物理内存简单分为三个区域:ZONE_NORMAL、ZONE_DMA、ZONE_HIGHMEM
vmalloc我们看到它是默认从ZONE_HIGMEM里申请,但是这两个函数虚拟地址是保持一致的,即都占用了4G地址空间的内核虚拟地址.通过上面的图,
我们确定了虚拟地址从哪里分配,以及对于的物理空间从哪里分配。
下面看看 vmalloc核心实现:
-
/**
-
* __vmalloc_node_range - allocate virtually contiguous memory
-
* @size: allocation size
-
* @align: desired alignment
-
* @start: vm area range start
-
* @end: vm area range end
-
* @gfp_mask: flags for the page level allocator
-
* @prot: protection mask for the allocated pages
-
* @node: node to use for allocation or -1
-
* @caller: caller's return address
-
*
-
* Allocate enough pages to cover @size from the page level
-
* allocator with @gfp_mask flags. Map them into contiguous
-
* kernel virtual space, using a pagetable protection of @prot.
-
*/
-
void *__vmalloc_node_range(unsigned long size, unsigned long align,
-
unsigned long start, unsigned long end, gfp_t gfp_mask,
-
pgprot_t prot, int node, const void *caller)
-
{
-
struct vm_struct *area;
-
void *addr;
-
unsigned long real_size = size;
-
-
size = PAGE_ALIGN(size);
-
if (!size || (size >> PAGE_SHIFT) > totalram_pages)
-
goto fail;
-
-
area = __get_vm_area_node(size, align, VM_ALLOC | VM_UNLIST, // 分配虚拟地址空间 把vm_struct 和vm_area(红黑树机制)关联起来.
-
start, end, node, gfp_mask, caller);
-
if (!area)
-
goto fail;
-
-
addr = __vmalloc_area_node(area, gfp_mask, prot, node, caller); //计算需要申请的页面,申请page,然后修改页表完成映射.
-
if (!addr)
-
return NULL;
-
-
/*
-
* In this function, newly allocated vm_struct is not added
-
* to vmlist at __get_vm_area_node(). so, it is added here.
-
*/
-
insert_vmalloc_vmlist(area); //把vm_struct插入 全局vmlist链表
-
-
/*
-
* A ref_count = 3 is needed because the vm_struct and vmap_area
-
* structures allocated in the __get_vm_area_node() function contain
-
* references to the virtual address of the vmalloc'ed block.
-
*/
-
kmemleak_alloc(addr, real_size, 3, gfp_mask); //内存泄露追踪
-
-
return addr;
-
-
fail:
-
warn_alloc_failed(gfp_mask, 0,
-
"vmalloc: allocation failure: %lu bytes\n",
-
real_size);
-
return NULL;
-
}
它的基本实现思路很简单:
1. 分配虚拟地址空间
2.对虚拟地址空间进行页表映射
需要熟知 下面两个结构体:
struct vmap_area
-
struct vmap_area {
-
unsigned long va_start;
-
unsigned long va_end;
-
unsigned long flags;
-
struct rb_node rb_node; /* address sorted rbtree */
-
struct list_head list; /* address sorted list */
-
struct list_head purge_list; /* "lazy purge" list */
-
struct vm_struct *vm;
-
struct rcu_head rcu_head;
-
};
vm_struct *area :
-
struct vm_struct {
-
struct vm_struct *next;
-
void *addr;
-
unsigned long size;
-
unsigned long flags;
-
struct page **pages;
-
unsigned int nr_pages;
-
phys_addr_t phys_addr;
-
const void *caller;
-
};
这里在说明一下vmalloc_init的初始化.
-
/*
-
* Set up kernel memory allocators
-
*/
-
static void __init mm_init(void)
-
{
-
/*
-
* page_cgroup requires contiguous pages,
-
* bigger than MAX_ORDER unless SPARSEMEM.
-
*/
-
page_cgroup_init_flatmem();
-
mem_init();
-
kmem_cache_init();
-
percpu_init_late();
-
pgtable_cache_init();
-
vmalloc_init();
-
}
其实在讲slab机制的时候已经说过。
-
void __init vmalloc_init(void)
-
{
-
struct vmap_area *va;
-
struct vm_struct *tmp;
-
int i;
-
-
for_each_possible_cpu(i) {
-
struct vmap_block_queue *vbq;
-
-
vbq = &per_cpu(vmap_block_queue, i);
-
spin_lock_init(&vbq->lock);
-
INIT_LIST_HEAD(&vbq->free);
-
}
-
-
/* Import existing vmlist entries. */
-
for (tmp = vmlist; tmp; tmp = tmp->next) { // 在系统启动或者初始化之初,vmlist为空.
-
va = kzalloc(sizeof(struct vmap_area), GFP_NOWAIT);
-
va->flags = VM_VM_AREA;
-
va->va_start = (unsigned long)tmp->addr;
-
va->va_end = va->va_start + tmp->size;
-
va->vm = tmp;
-
__insert_vmap_area(va);
-
}
-
-
vmap_area_pcpu_hole = VMALLOC_END;
-
-
vmap_initialized = true;
-
}
下面就说说__get_vm_area_node函数:
-
static struct vm_struct *__get_vm_area_node(unsigned long size,
-
unsigned long align, unsigned long flags, unsigned long start,
-
unsigned long end, int node, gfp_t gfp_mask, const void *caller)
-
{
-
struct vmap_area *va;
-
struct vm_struct *area;
-
-
BUG_ON(in_interrupt());
-
if (flags & VM_IOREMAP) { // ioremap标志,映射的是设备内存
-
int bit = fls(size);
-
-
if (bit > IOREMAP_MAX_ORDER)
-
bit = IOREMAP_MAX_ORDER;
-
else if (bit < PAGE_SHIFT)
-
bit = PAGE_SHIFT;
-
-
align = 1ul << bit;
-
}
-
-
size = PAGE_ALIGN(size);
-
if (unlikely(!size))
-
return NULL;
-
-
area = kzalloc_node(sizeof(*area), gfp_mask & GFP_RECLAIM_MASK, node);
-
if (unlikely(!area))
-
return NULL;
-
-
/*
-
* We always allocate a guard page.
-
*/
-
size += PAGE_SIZE; // 多偏移一页,为了防止访问越界,由于多出来的一页并不映射,所以当访问的时候,会引发保护异常.
-
-
va = alloc_vmap_area(size, align, start, end, node, gfp_mask); // 申请vm_area虚拟地址空间
-
if (IS_ERR(va)) {
-
kfree(area);
-
return NULL;
-
}
-
-
/*
-
* When this function is called from __vmalloc_node_range,
-
* we do not add vm_struct to vmlist here to avoid
-
* accessing uninitialized members of vm_struct such as
-
* pages and nr_pages fields. They will be set later.
-
* To distinguish it from others, we use a VM_UNLIST flag.
-
*/
-
if (flags & VM_UNLIST) // 必然走这里
-
setup_vmalloc_vm(area, va, flags, caller); // 关联vm_struct 和 vm_area
-
else
-
insert_vmalloc_vm(area, va, flags, caller);
-
-
return area;
-
}
这个函数核心就是alloc_vmap_area,这个很有趣的,之前我们讲到了vmalloc申请的虚拟地址范围,而它只传递了size而已,对于mips,x86,arm会有不同的虚拟空间.
-
/*
-
* Allocate a region of KVA of the specified size and alignment, within the
-
* vstart and vend.
-
*/
-
static struct vmap_area *alloc_vmap_area(unsigned long size,
-
unsigned long align,
-
unsigned long vstart, unsigned long vend,
-
int node, gfp_t gfp_mask)
-
{
-
struct vmap_area *va;
-
struct rb_node *n;
-
unsigned long addr;
-
int purged = 0;
-
struct vmap_area *first;
-
-
BUG_ON(!size);
-
BUG_ON(size & ~PAGE_MASK);
-
BUG_ON(!is_power_of_2(align));
-
-
va = kmalloc_node(sizeof(struct vmap_area),
-
gfp_mask & GFP_RECLAIM_MASK, node);
-
if (unlikely(!va))
-
return ERR_PTR(-ENOMEM);
-
-
retry:
-
spin_lock(&vmap_area_lock);
-
/*
-
* Invalidate cache if we have more permissive parameters.
-
* cached_hole_size notes the largest hole noticed _below_
-
* the vmap_area cached in free_vmap_cache: if size fits
-
* into that hole, we want to scan from vstart to reuse
-
* the hole instead of allocating above free_vmap_cache.
-
* Note that __free_vmap_area may update free_vmap_cache
-
* without updating cached_hole_size or cached_align.
-
*/
-
if (!free_vmap_cache || //第一次调用的时候 free_vmap_cache为空,后来即后边的代码line 105 : free_vmap_cache = &va->rb_node; 一般不为空 ;一般会发 // 生align < cached_align的情况,即会清除free_vmap_cache。有时候align比较大的时候,它会跳过一段虚拟地址空间.后面的申请由于没 //有free_vmap_cache,所以它需要重新查询
-
size < cached_hole_size ||
-
vstart < cached_vstart ||
-
align < cached_align) {
-
nocache:
-
cached_hole_size = 0;
-
free_vmap_cache = NULL;
-
}
-
/* record if we encounter less permissive parameters */
-
cached_vstart = vstart;
-
cached_align = align;
-
-
/* find starting point for our search */
-
if (free_vmap_cache) { // 第一次使用的时候为空;当不为空时,它保持上次申请的节点,并初始化addr为va_end.
-
first = rb_entry(free_vmap_cache, struct vmap_area, rb_node);
-
addr = ALIGN(first->va_end, align);
-
if (addr < vstart)
-
goto nocache;
-
if (addr + size - 1 < addr)
-
goto overflow;
-
-
} else {
-
addr = ALIGN(vstart, align);
-
if (addr + size - 1 < addr)
-
goto overflow;
-
-
n = vmap_area_root.rb_node; // 同样vmap_area_root.rb_node; 初始化也为空,第一次使用为空
-
first = NULL;
-
-
while (n) { // 当不是第一申请,并且free_cache为空的时候, 需要重新找到根节点即va_start <= addr
-
struct vmap_area *tmp;
-
tmp = rb_entry(n, struct vmap_area, rb_node);
-
-
if (tmp->va_end >= addr) {
-
first = tmp;
-
if (tmp->va_start <= addr)
-
break;
-
n = n->rb_left;
-
} else
-
n = n->rb_right;
-
}
-
-
if (!first)
-
goto found;
-
}
-
-
/* from the starting point, walk areas until a suitable hole is found */
-
while (addr + size > first->va_start && addr + size <= vend) { // 当不是第一申请,并且free_cache为空的时候,查询红黑树节点,找到合适的空间地址.
-
if (addr + cached_hole_size < first->va_start)
-
cached_hole_size = first->va_start - addr;
-
addr = ALIGN(first->va_end, align);
-
if (addr + size - 1 < addr)
-
goto overflow;
-
-
if (list_is_last(&first->list, &vmap_area_list)) // 默认不会在这里操作。也就是说它没有元素.
-
goto found;
-
-
first = list_entry(first->list.next,
-
struct vmap_area, list);
-
}
-
-
found:
-
if (addr + size > vend)
-
goto overflow;
-
-
va->va_start = addr;
-
va->va_end = addr + size;
-
va->flags = 0;
-
__insert_vmap_area(va); // 添加到红黑树 vmap_area_root
-
free_vmap_cache = &va->rb_node; // 初始化free_vmap_cache ,它会影响后续虚拟空间的申请.
-
spin_unlock(&vmap_area_lock);
-
-
BUG_ON(va->va_start & (align-1));
-
BUG_ON(va->va_start < vstart);
-
BUG_ON(va->va_end > vend);
-
-
return va;
-
-
overflow:
-
spin_unlock(&vmap_area_lock);
-
if (!purged) {
-
purge_vmap_area_lazy();
-
purged = 1;
-
goto retry;
-
}
-
if (printk_ratelimit())
-
printk(KERN_WARNING
-
"vmap allocation for size %lu failed: "
-
"use vmalloc= to increase size.\n", size);
-
kfree(va);
-
return ERR_PTR(-EBUSY);
-
}
既然我们已经开辟了虚拟地址空间,那么还需要做的当然是和页面一一映射起来.
看函数__vmalloc_area_node:
-
static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
-
pgprot_t prot, int node, const void *caller)
-
{
-
const int order = 0;
-
struct page **pages;
-
unsigned int nr_pages, array_size, i;
-
gfp_t nested_gfp = (gfp_mask & GFP_RECLAIM_MASK) | __GFP_ZERO;
-
-
nr_pages = (area->size - PAGE_SIZE) >> PAGE_SHIFT; //申请多少pages
-
array_size = (nr_pages * sizeof(struct page *)); //需要多大的存放page指针的空间 .
-
-
area->nr_pages = nr_pages;
-
/* Please note that the recursion is strictly bounded. */
-
if (array_size > PAGE_SIZE) { // 这里默认page_size 为4k 即4096 ,地址32位的话,相当于申请1024个pages:4M空间
-
pages = __vmalloc_node(array_size, 1, nested_gfp|__GFP_HIGHMEM,
-
PAGE_KERNEL, node, caller);
-
area->flags |= VM_VPAGES;
-
} else {
-
pages = kmalloc_node(array_size, nested_gfp, node); // 小于一页,则直接利用slab机制申请物理空间地址 给pages.
-
}
-
area->pages = pages;
-
area->caller = caller;
-
if (!area->pages) {
-
remove_vm_area(area->addr);
-
kfree(area);
-
return NULL;
-
}
-
-
for (i = 0; i < area->nr_pages; i++) { // 每次申请一个page利用alloc_page直接申请物理页面
-
struct page *page;
-
gfp_t tmp_mask = gfp_mask | __GFP_NOWARN;
-
-
if (node < 0)
-
page = alloc_page(tmp_mask);
-
else
-
page = alloc_pages_node(node, tmp_mask, order);
-
-
if (unlikely(!page)) {
-
/* Successfully allocated i pages, free them in __vunmap() */
-
area->nr_pages = i;
-
goto fail;
-
}
-
area->pages[i] = page; // 分配的地址存放在指针数组.
-
}
-
-
if (map_vm_area(area, prot, &pages)) // 修改页表 ,一页一页的实现映射,以及flush cache保持数据的一致性;对页面映射和操作感兴趣的可以深入看看这个函数.
-
goto fail;
-
return area->addr;
-
-
fail:
-
warn_alloc_failed(gfp_mask, order,
-
"vmalloc: allocation failure, allocated %ld of %ld bytes\n",
-
(area->nr_pages*PAGE_SIZE), area->size);
-
vfree(area->addr);
-
return NULL;
-
}
而insert_vmalloc_vmlist很明显把vm_struct插入到vmlist。
那么就完成了整个过程,没有想象的复杂,当然对内存有了更多的认识,这里还需要说一下,一般情况下有高端内存会比没有的好些,防止了vmalloc申请的时候造成的TLB抖动等问题,更少的破坏normal空间。
可以通过proc来查看vmalloc的一下信息:
-
cat /proc/vmallocinfo
-
0xc0002000-0xc0045000 274432 jffs2_zlib_init+0x24/0xa4 pages=66 vmalloc
-
0xc0045000-0xc0051000 49152 jffs2_zlib_init+0x40/0xa4 pages=11 vmalloc
-
0xc0051000-0xc0053000 8192 brcmnand_create_cet+0x244/0x788 pages=1 vmalloc
-
0xc0053000-0xc0055000 8192 ebt_register_table+0x98/0x39c pages=1 vmalloc
还有:
-
# cat /proc/vmstat
-
#cat /proc/meminfo
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