Stackable file systems and NFS

[Ranjan Sinha]

Hi,

We are writing a stackable file system. While testing this on NFS we
came across a situation where sometimes we are getting ASCII NUL bytes
in the buffer read from the lower file system (NFS in this case). This
happens when NFS client 1 is writing to a file and NFS client 2 is
continuously reading from the same file. A simple case like the
following also shows the same problem

----------------8<------------------8<-----------------------------
while (1)
    {
        if (vfs_getattr(filp->f_vfsmnt, filp->f_dentry, &stat) != 0)
        {
            printk(KERN_INFO "Error in getting attribute\n");
            break;
        }

        if (no_inc == 60)
        {
            printk(KERN_INFO "File not increased for 1 min. Quitting\n");
            break;
        }

        inc = stat.size - old_size;
        if (inc)
        {
            old_size = stat.size;
            no_inc = 0;
            count = vfs_read(filp, buf,
                                  sizeof(buf), &filp->f_pos);
            if (count < 0)
            {
                 printk(KERN_INFO "Error in reading file\n");
                 goto out;
            }
            printk(KERN_INFO "File increase = [%llu], total read = [%llu]"
                             " file_len = [%llu] File size from inode
= [%llu]\n",
                             inc, count, stat.size,
                             i_size_read(filp->f_dentry->d_inode));
            printk(KERN_INFO
"----------------------------------------------------\n");
            debug_dump("Read buffer", buf, total_count);
        }
        else
        {
            no_inc++;
        }
}
----------------8<------------------8<-----------------------------

We have also noticed that the expected increase (inc) and the size
returned in (vfs_read()) is different. A similar program in user space
also reproduces this issue. A google search for this issue tells me
that others have seen this issue as well.
http://stackoverflow.com/questions/6814404/java-inputstream-read-methods-returning-ascii-nul-characters-for-file-in-a-nfs

Is this a known and expected behaviour of NFS?

PS : This is a very old kernel (2.6.18.274.) We still have to support RHEL 5 :)

-- 
Regards,
Ranjan