1. 简介
对于mmap在用户态通过函数以下函数进行调用:
void* mmap( void* addr, size_t size, int prot, int flags, int fd, long offset )
然后进入系统调用。
2. Kernel mmap实现
1）然后进入系统调用，其系统调用号为:
kernel/arch/arm/include/asm/unistd.h
#define __NR_mmap2 (__NR_SYSCALL_BASE+192)
2）触发软中断
其ISR 代码位于kernel/arch/arm/kernel/entry-common.S的ENTRY(vector_swi), __NR_mmap2对应的函数为:sys_mmap2(位于linux/arch/arm/kernel/calls.S)
3）sys_mmap2的实现
位于kernel/arch/arm/kernel/entry-common.S,实现代码如下：
/* * Note: off_4k (r5) is always units of 4K. If we can't do the requested * offset, we return EINVAL. */ sys_mmap2: #if PAGE_SHIFT > 12 tst r5, #PGOFF_MASK moveq r5, r5, lsr #PAGE_SHIFT - 12 streq r5, [sp, #4] beq sys_mmap_pgoff mov r0, #-EINVAL mov pc, lr #else str r5, [sp, #4] b sys_mmap_pgoff #endif
4） 调用sys_mmap_pgoff
在kernel/include/linux/syscalls.h中定义如下：
asmlinkage long sys_mmap_pgoff(unsigned long addr, unsigned long len, unsigned long prot, unsigned long flags, unsigned long fd, unsigned long pgoff);
6）sys_mmap_pgoff实现
在kernel/mm/mmap.c中实现如下：
SYSCALL_DEFINE6(mmap_pgoff, unsigned long, addr, unsigned long, len,
unsigned long, prot, unsigned long, flags,
unsigned long, fd, unsigned long, pgoff)
{
struct file *file = NULL;
unsigned long retval = -EBADF;
if (!(flags & MAP_ANONYMOUS)) {
audit_mmap_fd(fd, flags);
if (unlikely(flags & MAP_HUGETLB))
return -EINVAL;
file = fget(fd);
if (!file)
goto out;
} else if (flags & MAP_HUGETLB) {
struct user_struct *user = NULL;
/*
* VM_NORESERVE is used because the reservations will be
* taken when vm_ops->mmap() is called
* A dummy user value is used because we are not locking
* memory so no accounting is necessary
*/
len = ALIGN(len, huge_page_size(&default_hstate));
file = hugetlb_file_setup(HUGETLB_ANON_FILE, len, VM_NORESERVE,
&user, HUGETLB_ANONHUGE_INODE);
if (IS_ERR(file))
return PTR_ERR(file);
}
flags &= ~(MAP_EXECUTABLE | MAP_DENYWRITE);
down_write(¤t->mm->mmap_sem);
retval = do_mmap_pgoff(file, addr, len, prot, flags, pgoff);
up_write(¤t->mm->mmap_sem);
if (file)
fput(file);
out:
return retval;
}
其功能为：从当前进程中获取用户态可用的虚拟地址空间（vm_area_struct *vma），在mmap_region中真正获取vma，然后调用file->f_op->mmap(file, vma)，调用具体的支持mmap的驱动来处理。
下面以binder驱动为例。
3. binder mmap实现
binder驱动的mmap函数为：binder_mmap，其实现代码如下：
static int binder_mmap(struct file *filp, struct vm_area_struct *vma)
{
int ret;
struct vm_struct *area;
struct binder_proc *proc = filp->private_data;
const char *failure_string;
struct binder_buffer *buffer;
if ((vma->vm_end - vma->vm_start) > SZ_4M)
vma->vm_end = vma->vm_start + SZ_4M;
binder_debug(BINDER_DEBUG_OPEN_CLOSE,
"binder_mmap: %d %lx-%lx (%ld K) vma %lx pagep %lx\n",
proc->pid, vma->vm_start, vma->vm_end,
(vma->vm_end - vma->vm_start) / SZ_1K, vma->vm_flags,
(unsigned long)pgprot_val(vma->vm_page_prot));
if (vma->vm_flags & FORBIDDEN_MMAP_FLAGS) {
ret = -EPERM;
failure_string = "bad vm_flags";
goto err_bad_arg;
}
vma->vm_flags = (vma->vm_flags | VM_DONTCOPY) & ~VM_MAYWRITE;
if (proc->buffer) {
ret = -EBUSY;
failure_string = "already mapped";
goto err_already_mapped;
}
area = get_vm_area(vma->vm_end - vma->vm_start, VM_IOREMAP);
if (area == NULL) {
ret = -ENOMEM;
failure_string = "get_vm_area";
goto err_get_vm_area_failed;
}
proc->buffer = area->addr;
proc->user_buffer_offset = vma->vm_start - (uintptr_t)proc->buffer;
#ifdef CONFIG_CPU_CACHE_VIPT
if (cache_is_vipt_aliasing()) {
while (CACHE_COLOUR((vma->vm_start ^ (uint32_t)proc->buffer))) {
printk(KERN_INFO "binder_mmap: %d %lx-%lx maps %p bad alignment\n", proc->pid, vma->vm_start, vma->vm_end, proc->buffer);
vma->vm_start += PAGE_SIZE;
}
}
#endif
proc->pages = kzalloc(sizeof(proc->pages[0]) * ((vma->vm_end - vma->vm_start) / PAGE_SIZE), GFP_KERNEL);
if (proc->pages == NULL) {
ret = -ENOMEM;
failure_string = "alloc page array";
goto err_alloc_pages_failed;
}
proc->buffer_size = vma->vm_end - vma->vm_start;
vma->vm_ops = &binder_vm_ops;
vma->vm_private_data = proc;
if (binder_update_page_range(proc, 1, proc->buffer, proc->buffer + PAGE_SIZE, vma)) {
ret = -ENOMEM;
failure_string = "alloc small buf";
goto err_alloc_small_buf_failed;
}
buffer = proc->buffer;
INIT_LIST_HEAD(&proc->buffers);
list_add(&buffer->entry, &proc->buffers);
buffer->free = 1;
binder_insert_free_buffer(proc, buffer);
proc->free_async_space = proc->buffer_size / 2;
barrier();
proc->files = get_files_struct(current);
proc->vma = vma;
/*printk(KERN_INFO "binder_mmap: %d %lx-%lx maps %p\n",
proc->pid, vma->vm_start, vma->vm_end, proc->buffer);*/
return 0;
err_alloc_small_buf_failed:
kfree(proc->pages);
proc->pages = NULL;
err_alloc_pages_failed:
vfree(proc->buffer);
proc->buffer = NULL;
err_get_vm_area_failed:
err_already_mapped:
err_bad_arg:
printk(KERN_ERR "binder_mmap: %d %lx-%lx %s failed %d\n",
proc->pid, vma->vm_start, vma->vm_end, failure_string, ret);
return ret;
}
1）获取kernel态虚拟地址空间：
struct vm_struct *area;
area = get_vm_area(vma->vm_end - vma->vm_start, VM_IOREMAP);
根据传过来的vma（数据结构为vm_area_struct,属于进程的一段空间，用于与内核空间映射用的），调用get_vm_area在内核的vmalloc区域获得一个相同大小的连续空间，数据结构为vm_struct,同时将该结构加入到vm_list统一管理
2）保存kernel态虚拟地址空间的起始地址，以便后面使用：
proc->buffer = area->addr;
3) 计算并保存进程用户态虚拟地址空间起始地址与kernel态虚拟地址空间的起始地址的差值， 以便后面使用。
proc->user_buffer_offset = vma->vm_start - (uintptr_t)proc->buffer;
4）分配物理页表项(struct page)
proc->pages = kzalloc(sizeof(proc->pages[0]) * ((vma->vm_end - vma->vm_start) / PAGE_SIZE), GFP_KERNEL);
5）binder_update_page_range
它的工作为：
a）分配物理页
b）分别对vma用户空间建立页表、对vmalloc区域建立页表映射关系。
前面有了用户态和Kernel态的虚拟地址空间，但是还不能访问，因为还没有对应的物理内存。
补充知识：
a）struct page用于跟踪描述一个物理页面是否正在被使用。所有的page结构将都被存入一个叫做mem_map的全局数组中.
b）在每个进程的task_struct中包含一个指向mm_struct结构的指针.进程的mm_struct中则包含了进程可执行影像的页目录指针pgd.还包含了指向vm_area_struct的几个指针,每个vm_area_struct包含一个进程的虚拟地址区域.
binder_update_page_range(proc, 1, proc->buffer, proc->buffer + PAGE_SIZE, vma)
proc->buffer指向内核的vmalloc 区域的起始地址，前面已经有了vma(vm_area_struct)和 area(vm_struct)。binder_update_page_range实现代码如下：
static int binder_update_page_range(struct binder_proc *proc, int allocate,
void *start, void *end,
struct vm_area_struct *vma)
{
void *page_addr;
unsigned long user_page_addr;
struct vm_struct tmp_area;
struct page **page;
struct mm_struct *mm;
binder_debug(BINDER_DEBUG_BUFFER_ALLOC,
"binder: %d: %s pages %p-%p\n", proc->pid,
allocate ? "allocate" : "free", start, end);
if (end <= start)
return 0;
if (vma)
mm = NULL;
else
mm = get_task_mm(proc->tsk);
if (mm) {
down_write(&mm->mmap_sem);
vma = proc->vma;
}
if (allocate == 0)
goto free_range;
if (vma == NULL) {
printk(KERN_ERR "binder: %d: binder_alloc_buf failed to "
"map pages in userspace, no vma\n", proc->pid);
goto err_no_vma;
}
for (page_addr = start; page_addr < end; page_addr += PAGE_SIZE) {
int ret;
struct page **page_array_ptr;
page = &proc->pages[(page_addr - proc->buffer) / PAGE_SIZE];
BUG_ON(*page);
//分配一个物理页
*page = alloc_page(GFP_KERNEL | __GFP_ZERO);
if (*page == NULL) {
printk(KERN_ERR "binder: %d: binder_alloc_buf failed "
"for page at %p\n", proc->pid, page_addr);
goto err_alloc_page_failed;
}
tmp_area.addr = page_addr;
tmp_area.size = PAGE_SIZE + PAGE_SIZE /* guard page? */;
page_array_ptr = page;
//根据kernel态的虚拟地址，分配对应的pud, pmd和pte并填充对应的值
//以使根据虚拟地址，可以通过pgd, pud, pmd和pte寻址到对应的物理存储单元
ret = map_vm_area(&tmp_area, PAGE_KERNEL, &page_array_ptr);
if (ret) {
printk(KERN_ERR "binder: %d: binder_alloc_buf failed "
"to map page at %p in kernel\n",
proc->pid, page_addr);
goto err_map_kernel_failed;
}
user_page_addr =
(uintptr_t)page_addr + proc->user_buffer_offset;
//根据用户态的虚拟地址，插入一页到用户空间的vma，
//从而用户空间访问从user_page_addr开始的一页内存时，
//从而可以访问到与page对应的物理页中对应的存储单元
ret = vm_insert_page(vma, user_page_addr, page[0]);
if (ret) {
printk(KERN_ERR "binder: %d: binder_alloc_buf failed "
"to map page at %lx in userspace\n",
proc->pid, user_page_addr);
goto err_vm_insert_page_failed;
}
/* vm_insert_page does not seem to increment the refcount */
}
if (mm) {
up_write(&mm->mmap_sem);
mmput(mm);
}
return 0;
free_range:
for (page_addr = end - PAGE_SIZE; page_addr >= start;
page_addr -= PAGE_SIZE) {
page = &proc->pages[(page_addr - proc->buffer) / PAGE_SIZE];
if (vma)
zap_page_range(vma, (uintptr_t)page_addr +
proc->user_buffer_offset, PAGE_SIZE, NULL);
err_vm_insert_page_failed:
unmap_kernel_range((unsigned long)page_addr, PAGE_SIZE);
err_map_kernel_failed:
__free_page(*page);
*page = NULL;
err_alloc_page_failed:
;
}
err_no_vma:
if (mm) {
up_write(&mm->mmap_sem);
mmput(mm);
}
return -ENOMEM;
}
a） map_vm_area: 映射Kernel虚拟地址到物理内存，为vmalloc 区域的连续地址空间进行页表映射，当然需要vm_struct （提供虚拟地址）参数和 page参数（用来make pte的），这就完成了内核区的映射
b） vm_insert_page: 更新vma对应的页表，这样就是实现了mmap功能
c）binder_update_page_range(proc, 1, proc->buffer, proc->buffer + PAGE_SIZE, vma)调用的时候只分配了1页，这个是为了节约空间，按需分配。而进程虚拟空间和vmalloc内核空间按需要分配，反正它不占用实际物理内存，所以开始就占用了所需的全部空间，而实际的物理页按需获取；
proc->vma为调用进程的一段用户空间；
proc->files为调用进程的files_struct结构；
proc->buffer_size为需要映射的长度（小于4m）-sizeof（struct binder_buffer）;
proc->pages为分配的物理页page的指针数组，开始只有一项，即1页，但是长度还是预留好了；
proc->buffer为内核连续映射区首地址 ；
proc->user_buffer_offset 为用户空间映射区首地址-内核空间连续映射的首地址。