On Wed, 28 Jan 2009 13:33:22 -0800 Greg KH <greg@xxxxxxxxx> 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@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ > -- 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
