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

Robert Hancock
2009-01-29T00:42:21Z

Greg KH wrote:
> 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.

Well, it sounds like a bug to me.. even if it's not supported, if you do
such an access, surely the kernel should detect that and return EINVAL
or something rather than reading corrupted data..