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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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