[RFC v14-rc2][PATCH 18/29] Restore open pipes

[Serge E. Hallyn]

Quoting Oren Laadan (orenl at cs.columbia.edu):
> When seeing a CR_FD_PIPE file type, we create a new pipe and thus
> have two file pointers (read- and write- ends). We only use one of
> them, depending on which side was checkpointed first. We register the
> file pointer of the other end in the hash table, with the 'objref'
> given for this pipe from the checkpoint, deposited for later use. At
> this point we also restore the contents of the pipe buffers.
> 
> When the other end arrives, it will have file type CR_FD_OBJREF. We
> will then use the corresponding 'objref' to retrieve the file pointer
> from the hash table, and attach it to the process.
> 
> Note the difference from the checkpoint logic: during checkpoint we
> placed the _inode_ of the pipe in the hash table, while during restart
> we place the resulting _file_ in the hash table.
> 
> We restore the pipe contents we manually allocation and attaching
> buffers to the pipe; (alternatively we could read the data from the
> image file and then write it into the pipe, or use splice() syscall).
> 
> Changelog[v14]:
>   - Discard the 'h.parent' field
>   - Check whether calls to cr_hbuf_get() fail
> 
> Signed-off-by: Oren Laadan <orenl at cs.columbia.edu>

Acked-by: Serge Hallyn <serue at us.ibm.com>

...

> +/* restore a pipe */
> +static int cr_read_fd_pipe(struct cr_ctx *ctx, struct cr_hdr_file *hh)
> +{
> +	struct file *file;
> +	int fds[2], which, ret;
> +
> +	file = cr_obj_get_by_ref(ctx, hh->fd_objref, CR_OBJ_FILE);
> +	if (IS_ERR(file))
> +		return PTR_ERR(file);
> +	else if (file)
> +		return cr_attach_get_file(file);

I think the casual reader would be helped by a comment like:

	/*
	 * if cr_obj_get_by_ref returned a file, then one end
	 * of the pipe has been restored, so we have
	 * cr_attach_get_file() attach the other end to a new
	 * fd, and we return that fd.
	 */

> +
> +	/* first encounter of this pipe: create it */
> +	ret = do_pipe(fds);
> +	if (ret < 0)
> +		return ret;

thanks,
-serge