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

KAMEZAWA Hiroyuki
2009-01-29T05:15:00Z

On Wed, 28 Jan 2009 13:33:22 -0800
Greg KH <greg@kroah.com> 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.)
>

IIUC, it's not related to 512bytes boundary. Just a race between
direct-io v.s. copy-on-write. Copy-on-Write while reading a page via DIO
is a problem.

Maybe it's true that if buffer is aligned to page size, no copy-on-write will
happen in usual program. But assuming HugeTLB page, which does Copy-on-Write,
data corruption will happen again. HugeTLB aligned buffer is nonsense.

Thanks,
-Kame

> 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.
>
> thanks,
>
> greg k-h
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at http://www.tux.org/lkml/
>