On Thu, Jan 29, 2009 at 10:40:22AM -0500, Jeff Moyer wrote: > 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. Thanks for the pointers to that, I'll follow along with it. greg k-h -- 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
