Re: fs requirements for loop device?

[Andrew Morton <akpm@zip.com.au>]

Urban Widmark wrote:
> 
> On Mon, 13 May 2002, Andrew Morton wrote:
> 
> > Can anyone point at a reason why loop shouldn't use the
> > ->read() and ->write() methods of the backing file's
> > file_operations?
> 
> The patch below is my naive implementation of that for 2.4.18.
> 
> ...

 loop.c |  113 +++++++++++------------------------------------------------------
 1 files changed, 20 insertions, 93 deletions

that's a good sign ;)

> ...
> +       fs = get_fs();
> +       set_fs(get_ds());
> +       ret = file->f_op->write(file, data, len, &pos);
> +       set_fs(fs);

f_op->write() returns the number of bytes written, or a negative
errno.   So you'll need

	if (ret > 0)
		ret = 0;

in there.

Also it may be nicer to copy `pos' into a local when you
call ->read and ->write, just to avoid altering your callers'
locals, and potentially surprising them (it's OK at present,
so minor point).

> ...
> +       /* This looks strange, if someone free's lo won't they also fput
> +          lo_backing_file? */
>         spin_lock_irq(&lo->lo_lock);
>         file = lo->lo_backing_file;
>         spin_unlock_irq(&lo->lo_lock);

They do.  loop_clr_fd() does fput().

If this code is trying to actually do something legitimate
then clearly it is doing it wrongly.  Just delete the locking
I think.

This is a big improvement to loop.  Let's try to get
this into 2.4.20.


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