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

    Cheers,
    Jeff


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