open(2) says O_DIRECT works on 512 byte boundries?

Greg KH
2009-01-28T21:34:53Z

In looking at open(2), it says that O_DIRECT works on 512 byte boundries
with the 2.6 kernel release:
Under Linux 2.4, transfer sizes, and the alignment of the user
buffer and the file offset must all be multiples of the logical
block size of the file system. Under Linux 2.6, alignment to
512-byte boundaries suffices.

However if you try to access an O_DIRECT opened file with a buffer that
is PAGE_SIZE aligned + 512 bytes, it fails in a bad way (wrong data is
read.)

Is this just a mistake in the documentation? Or am I reading it
incorrectly?

I have a test program that shows this if anyone wants it.

thanks,

greg k-h