这允许把用户内存分割为更多的段。
dxfer_len：数据传输的用户内存的长度。
现在，让我们来研究一下用户操作用户内存的内核api。
Now, let's explore the kernel APIs for manipulating user memory.
用户内存被清除时的延迟将因系统和内存大小的物理配置。
The delay for users while memory is cleared will vary by the physical configuration of the system and memory size.
dxferp：指向数据传输时长度至少为dxfer_len字节的用户内存的指针。
Dxferp: a pointer to user memory of at least dxfer_len bytes in length for data transfer.
如果用户内存不足以执行复杂计算，他们的计算机就会试图在磁盘上寻找空间来完成该计算。
If a user's memory is not sufficient to perform complex calculations, their computer will try to find space on the disk to complete the calculation.
如果用户内存不足以执行复杂计算，他们的计算机就会试图在磁盘上寻找空间来完成该计算。
If a user's memory is not sufficient to perform complex calculations, their computer will try to find space on the disk to complete the calculation.
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