Greg KH <greg@xxxxxxxxx> 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@xxxxxxxxx> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
