On Thu, 18 Aug 2011, Jeff Moyer wrote:
> Lukas Czerner <lczerner@xxxxxxxxxx> writes:
>
> >> @@ -484,6 +485,29 @@ static int do_bio_filebacked(struct loop_device *lo, struct bio *bio)
> >> }
> >> }
> >>
> >> + /*
> >> + * We use punch hole to reclaim the free space used by the
> >> + * image a.k.a. discard. However we do support discard if
> >> + * encryption is enabled, because it may give an attacker
> >> + * useful information.
> >> + */
> >> + if (bio->bi_rw & REQ_DISCARD) {
> >> + struct file *file = lo->lo_backing_file;
> >> + int mode = FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE;
> >> +
> >> + if ((!file->f_op->fallocate) ||
> >> + lo->lo_encrypt_key_size) {
> >> + ret = -EOPNOTSUPP;
> >> + goto out;
> >> + }
> >> + ret = file->f_op->fallocate(file, mode, pos,
> >> + bio->bi_size);
> >> + if (unlikely(ret && ret != -EINVAL &&
> >> + ret != -EOPNOTSUPP))
> >> + ret = -EIO;
> >> + goto out;
> >> + }
> >> +
>
> Seems you missed the bizarre case of configuring a loop device over top
> of a block device.
Wow, that is a bizarre case I did not think about at all. But since it
is so bizarre, do we even care ? The thing is that the only case where
it would make a difference is if the loop device is put on top of block
device which actually supports discard.
In order to fix that I would need to dig out the actual limits for that
device and set that appropriately for the loop device. Is that worth it
? It is not like someone will ever do that (or should) :).
Thanks!
-Lukas
>
> Cheers,
> Jeff
>
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