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

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Date:	Thu, 18 Aug 2011 21:12:30 +0200
From:	Jens Axboe <jaxboe@...ionio.com>
To:	Lukas Czerner <lczerner@...hat.com>
CC:	Milan Broz <mbroz@...hat.com>, Jeff Moyer <jmoyer@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"achender@...ux.vnet.ibm.com" <achender@...ux.vnet.ibm.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] loop: add discard support for loop devices

On 2011-08-18 21:08, Lukas Czerner wrote:
> On Thu, 18 Aug 2011, Milan Broz wrote:
> 
>> On 08/18/2011 05:49 PM, Lukas Czerner wrote:
>>> On Thu, 18 Aug 2011, Jeff Moyer wrote:
>>>> 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) :).
>>
>> It is bizarre (and being device-mapper developer I surely know better way :-)
>> but people are still using that.
>>
>> Historically one of the use of underlying block device was cryptoloop, but here
>> I think it should be completely deprecated (cryptsetup can handle all old loop
>> modes as well and default modes for cryptoloop are not safe).
>> [Can we finally remove crypto loop option it from kernel? ... ok, just tried:)]
>>
>> There is also out of tree loop-aes based on heavily patched loop device
>> which usually uses block device underneath
>> (cryptsetup already can handle all loop-aes modes as well).
>>
>> Sometimes it is used with --offset parameter for some reason
>> (like linear device-mapper mapping).
>>
>> So I do not care if you do not support discard here but please do not break
>> support for block device mapped through loop.
> 
> I do not think that this is the case with my patch. Also, as you know using
> discard on encrypted device is not a good idea.

It's not a bizarre use case at all, so would be nice to support like we
support anything else over a bdev as well. Your patch should not break
it, so looks fine.

Shall we queue it up for 3.2? It's a good way to beat on fs discard
support, fio could be easily configured for that.

-- 
Jens Axboe

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