O_DIRECT write to file by block-aligned, block-multiple buf fails?

[Scott McDermott <vaxerdec@frontiernet.net>]

I thought based on searching the archives that 2.4 O_DIRECT
requires fs block size alignment of the buffer, and that the
buffer is an exact multiple of block size.  This should mean
I can write aligned pages with direct IO, right?

        int
        main (void)
        {
                char *buf;
                int fd;

                buf = memalign(getpagesize(), getpagesize());
                fd = open("/tmp/testfile", O_TRUNC|O_WRONLY|O_DIRECT);
                if (write(fd, buf, getpagesize()) == -1)
                        perror("write");
        }

        $ ./test
        write: Invalid argument

        $ grep /tmp /proc/mounts
        /dev/hda5 /mnt/tmp ext3 rw 0 0

        $ sudo dumpe2fs /dev/hda5 | grep Block\ size
        dumpe2fs 1.27 (8-Mar-2002)
        Block size:               4096

        $ uname -rm
        2.4.21-pre4-ac1 i686

what silly thing am I not understanding here?