直接磁盘IO与缓冲区之间的直接IO由mmap分配

Question

I need help regarding to direct disk IO. I open a file by file descriptor (fd) with flag O_DIRECT. In my user space application, I want to read large amount of data from the file and these data was used once only. A piece of un-cached memory buffer was allocated in my kernel module through "set_memory_uc" (using x86) and "remap_pfn_range" with vm_page_prot set noncached (pgrot_noncached). This buffer is aim to be used for DMA transfer via PCIe.

I tried

read(fd, buffer, len)

and

lseek(fd, 0x1000, SEEK_SET)

'buffer' VA is aligned to 4k boundary. So does 'len' (n*4k)

for somehow ,'lseek'seems works because after calling lseek it returns 0x1000

but 'read' return -1

Is there any restriction for direct disk read disk data to a mmap buffer?

Answer 1

代替O_DIRECT ，考虑将posix_fadvise()与POSIX_FADV_NOREUSE标志一起使用，以指示“数据将仅使用一次”。