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

Greg KH
2009-01-30T06:19:15Z

On Thu, Jan 29, 2009 at 10:40:22AM -0500, Jeff Moyer wrote:
> Greg KH <greg@kroah.com> writes:
>
> > On Thu, Jan 29, 2009 at 03:59:12PM +1300, Michael Kerrisk wrote:
> >> On Thu, Jan 29, 2009 at 2:17 PM, Greg KH <greg@kroah.com> wrote:
> >> >
> >> >
> >> >
> >> > 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.
> >>
> >> Greg,
> >>
> >> Can you post your test program?
> >
> > Sure, here it is. I'm still not quite sure it is valid, but at first
> > glance it seems to be.
> >
> > Run it once with no arguments and all of the files will be created.
> > Then run it again with no offset being asked for:
> > ./dma_thread -a 0
> > then with an offset:
> > ./dma_thread -a 512
> >
> > The second one breaks.
>
> There are several folks working on this. See "Corruption with O_DIRECT
> and unaligned user buffers" on the linux-fsdevel list. There is also a
> Red Hat bugzilla for this (471613) that several folks have been working
> through.

Thanks for the pointers to that, I'll follow along with it.

greg k-h