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

Michael Kerrisk
2009-01-29T02:59:12Z

On Thu, Jan 29, 2009 at 2:17 PM, Greg KH <greg-U8xfFu+wG4EAvxtiuMwx3w@public.gmane.org> 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?

Cheers,

Michael

--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
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