On Fri, Oct 21, 2016 at 11:13 PM, Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
> On Fri, Oct 21, 2016 at 11:53:41AM +0300, Amir Goldstein wrote:
>> 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??
>
> Hmm.., Interesting. Will something like following work? This applies on
> top of Miklos's patch. It seems to work for me. It might be completely
> broken/racy though. Somebody who understands this code well, will have
> to have a look.
>
The idea sounded scary already when I suggested it :)
See below what I think is scary about this implementation...
Thanks for following through.
> ---
> fs/file.c | 41 +++++++++++++++++++++++++++++++++++++++++
> fs/overlayfs/inode.c | 1 +
> 2 files changed, 42 insertions(+)
>
> Index: rhvgoyal-linux/fs/overlayfs/inode.c
> ===================================================================
> --- rhvgoyal-linux.orig/fs/overlayfs/inode.c 2016-10-21 15:43:05.391488406 -0400
> +++ rhvgoyal-linux/fs/overlayfs/inode.c 2016-10-21 16:07:57.409420795 -0400
> @@ -416,6 +416,7 @@ static ssize_t ovl_read_iter(struct kioc
> if (IS_ERR(upperfile)) {
> ret = PTR_ERR(upperfile);
> } else {
> + replace_file(file, upperfile);
When fdtable is not shared (single threaded process), after this call
I think that file pointer
may be free (?), because file is not reference counted.
Although I did not see any code in VFS callers trying to dereference
the file pointer after
calling read_iter(), this seems like a dangerous practice, so will
need to a way to fix that.
> ret = vfs_iter_read(upperfile, to, &iocb->ki_pos);
> fput(upperfile);
> }
> Index: rhvgoyal-linux/fs/file.c
> ===================================================================
> --- rhvgoyal-linux.orig/fs/file.c 2016-10-21 15:43:05.391488406 -0400
> +++ rhvgoyal-linux/fs/file.c 2016-10-21 16:08:18.168420795 -0400
> @@ -864,6 +864,47 @@ Ebusy:
> return -EBUSY;
> }
>
> +
> +int replace_file(struct file *old_file, struct file *new_file)
> +{
> +#define MAX_TO_FREE 8
> + int n, idx = 0;
> + struct files_struct *files = current->files;
> + struct fdtable *fdt;
> + struct file *to_free[MAX_TO_FREE];
> + bool retry = false;
> +
> +try_again:
> + spin_lock(&files->file_lock);
> + for (n = 0, fdt = files_fdtable(files); n < fdt->max_fds; n++) {
> + struct file *file;
> + file = rcu_dereference_check_fdtable(files, fdt->fd[n]);
> + if (!file)
> + continue;
> + if (file == old_file) {
> + get_file(new_file);
> + rcu_assign_pointer(fdt->fd[n], new_file);
> + to_free[idx++] = file;
> + if (idx >= MAX_TO_FREE) {
> + retry = true;
> + break;
> + }
> + }
> + }
> + spin_unlock(&files->file_lock);
> + while (idx) {
> + filp_close(to_free[--idx], files);
> + }
> +
> + if (retry) {
> + retry = false;
> + idx = 0;
> + goto try_again;
> + }
> + return 0;
> +}
> +EXPORT_SYMBOL(replace_file);
> +
> int replace_fd(unsigned fd, struct file *file, unsigned flags)
> {
> int err;
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