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

Greg KH
2009-01-29T01:17:58Z

On Wed, Jan 28, 2009 at 06:41:49PM -0600, Robert Hancock wrote:
>
>
> 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..

It doesn't. It says the read is successful, yet the data is not really
read into the buffer. Portions of it is, but not the amount we asked
for.

thanks,

greg k-h