From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jeff Moyer <jmoyer@redhat.com>
Subject: Re: [PATCH] loop: add discard support for loop devices
Date: Thu, 18 Aug 2011 11:33:22 -0400
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Cc: linux-kernel@vger.kernel.org, achender@linux.vnet.ibm.com,
	Jens Axboe <jaxboe@fusionio.com>, linux-fsdevel@vger.kernel.org
To: Lukas Czerner <lczerner@redhat.com>
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In-Reply-To: <alpine.LFD.2.00.1108171511070.3699@dhcp-27-109.brq.redhat.com>
	(Lukas Czerner's message of "Wed, 17 Aug 2011 15:13:31 +0200 (CEST)")
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Lukas Czerner <lczerner@redhat.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.

Cheers,
Jeff