Quoting Oren Laadan (orenl@xxxxxxxxxxxxxxx):
> 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@xxxxxxxxxxxxxxx>
Acked-by: Serge Hallyn <serue@xxxxxxxxxx>
...
> +/* 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
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