接着上篇的讲。
我们注意到binder在使用buffer的时候一次声明一个proc（对应一个进程）的buffer总大小，然后分配一页并做好映射。当使用时，发现空间不足，我们接着映射，同时把这个buffer拆成两个，并把剩余的继续放到free_buffers里面。下面从binder_alloc_buf这个函数讲起：
static struct binder_buffer *binder_alloc_buf(struct binder_proc *proc,
size_t data_size, size_t offsets_size, int is_async)
{
struct rb_node *n = proc->free_buffers.rb_node;
struct binder_buffer *buffer;
size_t buffer_size;
struct rb_node *best_fit = NULL;
void *has_page_addr;
void *end_page_addr;
size_t size;
if (proc->vma == NULL) {
printk(KERN_ERR "binder: %d: binder_alloc_buf, no vma\n",
proc->pid);
return NULL;
}
size = ALIGN(data_size, sizeof(void *)) +
ALIGN(offsets_size, sizeof(void *));
if (size < data_size || size < offsets_size) {
binder_user_error("binder: %d: got transaction with invalid "
"size %zd-%zd\n", proc->pid, data_size, offsets_size);
return NULL;
}
if (is_async &&
proc->free_async_space < size + sizeof(struct binder_buffer)) {
if (binder_debug_mask & BINDER_DEBUG_BUFFER_ALLOC)
printk(KERN_ERR "binder: %d: binder_alloc_buf size %zd f"
"ailed, no async space left\n", proc->pid, size);
return NULL;
}
/*这个地方开始行动，在rb树上查找和请求的size一样大的buffer，size=data_size+offsets_size按照指针长度向上补齐*/
while (n) {
buffer = rb_entry(n, struct binder_buffer, rb_node);
BUG_ON(!buffer->free);
buffer_size = binder_buffer_size(proc, buffer);
if (size < buffer_size) {
best_fit = n;
n = n->rb_left;
} else if (size > buffer_size)
n = n->rb_right;
else {
best_fit = n;
break;
}
} if (best_fit == NULL) {
printk(KERN_ERR "binder: %d: binder_alloc_buf size %zd failed, "
"no address space\n", proc->pid, size);
return NULL;
}/*刚好匹配的概率是很低的哦，所以一般n=NULL，这个地方，buffer此时要不是最小的一个，就是最大的一个*/if (n == NULL) {buffer = rb_entry(best_fit, struct binder_buffer, rb_node);buffer_size = binder_buffer_size(proc, buffer);}/*buffer的终点，按照page向下对其，因为buffer的总大小只有data+buffer_size这么大，否则就超了*/has_page_addr =(void *)(((uintptr_t)buffer->data + buffer_size) & PAGE_MASK);
/*发现如果size比较大，那么就不分割了，具体的分割操作在后边进行。size + sizeof(struct binder_buffer)+4怎么理解呢，如果这个buffer空间能容下size并且还有一个binder_buffer剩下还多一点，那么就分割了，否则，少于一个binder_buffer结构体的空间也没啥用啊，因为放不了数据，在上一篇我们说到过，buffer控制部分和数据是放在一起的，只有头放不下数据的话也就没有意义。明白了吧。
*/
if (n == NULL) {
if (size + sizeof(struct binder_buffer) + 4 >= buffer_size)
buffer_size = size; /* no room for other buffers */
else
buffer_size = size + sizeof(struct binder_buffer);
}/*计算实际的映射终点，向上对齐*/
end_page_addr =
(void *)PAGE_ALIGN((uintptr_t)buffer->data + buffer_size);
if (end_page_addr > has_page_addr)
end_page_addr = has_page_addr;//上面计算end的时候可能过大，恢复到合适值/*从这个地方开始，就进入到上篇文章一样的操作呢，分配物理页，分别对vma（用户空间）和vm（内核空间）建立页表关联*/
if (binder_update_page_range(proc, 1,
(void *)PAGE_ALIGN((uintptr_t)buffer->data), end_page_addr, NULL)
)
return NULL;
rb_erase(best_fit, &proc->free_buffers);//从free上面删除掉并插入到已分配里面进行管理
buffer->free = 0;
binder_insert_allocated_buffer(proc, buffer);
if (buffer_size != size) {/*这个就是该不该拆分buffer的标志。相等的话说明已经容不下第二个buffer，虽然可以有小于44B的浪费，因为44B留着也没啥用的。把大buffer拆成小的buffer 接着放入free队列*/
struct binder_buffer *new_buffer = (void *)buffer->data + size;
list_add(&new_buffer->entry, &buffer->entry);
new_buffer->free = 1;
binder_insert_free_buffer(proc, new_buffer);
}
buffer->data_size = data_size;
buffer->offsets_size = offsets_size;
buffer->async_transaction = is_async;
if (is_async) {
proc->free_async_space -= size + sizeof(struct binder_buffer);
if (binder_debug_mask & BINDER_DEBUG_BUFFER_ALLOC_ASYNC)
printk(KERN_INFO "binder: %d: binder_alloc_buf size %zd "
"async free %zd\n", proc->pid, size,
proc->free_async_space);
}
return buffer;
}
log如下：
binder_open: 31:31,name=servicemanager
[ binder_mmap ] vonnyfly(lfeng^-^)~~~~~~~~~~~binder_buffer size=40
binder_mmap: 31 40009000-40029000 (128 K) vma 75 pagep 30f 获取vma区域
[ binder_mmap ] vonnyfly(lfeng^-^)~~~~~~~~~~~vma->vm_start=0x40009000,proc->buffer=0xc6840000
binder: 31: allocate pages c6840000-c6841000 这个地方，分配一个物理页
binder: 31: add free buffer, size 131032, at c6840000 添加到free链表里
binder: 31: buffer_size 1ffd8---buffer->data=c6840028,(uintptr_t)buffer->data + buffer_size=c6860000,has_page_addr=c6860000
binder: 31: binder_alloc_buf size 128 got buffer c6840000 buffer_size 168--data_size=124,offsets_size=4---buffer->data=c6840028,has_page_addr=c6860000,end_page_addr=c6841000
binder: 31: allocate pages c6841000-c6841000 已经在binder_free_buf里面了，从上面log看出，请求的size=128B，所以无需重新映射，之前mmap里面的一页够用了
binder: 31: add free buffer, size 130864, at c68400a8 把剩余的 130864B=131032 - 128 -40（头部）。首地址现在成了0xc6840000+128=0xc68400a8放入到free链表里。可以算一下：
binder: 31: binder_free_buf c6840000 size 128 buffer_size 128
binder: 31: free pages c6841000-c6840000
binder: 31: merge free, buffer c68400a8 share page with c6840000
binder: 31: add free buffer, size 131032, at c6840000
binder: 31: buffer_size 1ffd8---buffer->data=c6840028,(uintptr_t)buffer->data + buffer_size=c6860000,has_page_addr=c6860000
binder: 31: binder_alloc_buf size 116 got buffer c6840000 buffer_size 156--data_size=112,offsets_size=4---buffer->data=c6840028,has_page_addr=c6860000,end_page_addr=c6841000
binder: 31: allocate pages c6841000-c6841000
binder: 31: add free buffer, size 130876, at c684009c
binder: 31: binder_free_buf c6840000 size 116 buffer_size 116
binder: 31: free pages c6841000-c6840000
binder: 31: merge free, buffer c684009c share page with c6840000
binder: 31: add free buffer, size 131032, at c6840000
接下来是重头戏，看binder_ioctl怎么实现的，我们上面的函数又是怎么调用到的。（未完，待续）