On Thu, Oct 20, 2016 at 11:54 PM, Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
> On Thu, Oct 20, 2016 at 04:46:30PM -0400, Vivek Goyal wrote:
>
> [..]
>> > +static ssize_t ovl_read_iter(struct kiocb *iocb, struct iov_iter *to)
>> > +{
>> > + struct file *file = iocb->ki_filp;
>> > + bool isupper = OVL_TYPE_UPPER(ovl_path_type(file->f_path.dentry));
>> > + ssize_t ret = -EINVAL;
>> > +
>> > + if (likely(!isupper)) {
>> > + const struct file_operations *fop = ovl_real_fop(file);
>> > +
>> > + if (likely(fop->read_iter))
>> > + ret = fop->read_iter(iocb, to);
>> > + } else {
>> > + struct file *upperfile = filp_clone_open(file);
>> > +
>>
>> IIUC, every read of lower file will call filp_clone_open(). Looking at the
>> code of filp_clone_open(), I am concerned about the overhead of this call.
>> Is it significant? Don't want to be paying too much of penalty for read
>> operation on lower files. That would be a common case for containers.
>>
>
> Looks like I read the code in reverse. So if I open a file read-only,
> and if it has not been copied up, I will simply call read_iter() on
> lower filesystem. But if file has been copied up, then I will call
> filp_clone_open() and pay the cost. And this will continue till this
> file is closed by caller.
>
I wonder if that cost could be reduced by calling replace_fd() or
some variant of it to install the cloned file onto the rofd after the
first access??
> When file is opened again, by that time it is upper file and we will
> install real fop in file (instead of overlay fop).
>
> Vivek
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