linux-kernel - Re: [PATCH] loop: add discard support for loop devices

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Date:	Thu, 18 Aug 2011 17:49:12 +0200 (CEST)
From:	Lukas Czerner <lczerner@...hat.com>
To:	Jeff Moyer <jmoyer@...hat.com>
cc:	Lukas Czerner <lczerner@...hat.com>, linux-kernel@...r.kernel.org,
	achender@...ux.vnet.ibm.com, Jens Axboe <jaxboe@...ionio.com>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] loop: add discard support for loop devices

On Thu, 18 Aug 2011, Jeff Moyer wrote:

> Lukas Czerner <lczerner@...hat.com> 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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