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

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Message-ID: <x49r54ir0a9.fsf@segfault.boston.devel.redhat.com>
Date:	Thu, 18 Aug 2011 15:23:57 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Jens Axboe <jaxboe@...ionio.com>
Cc:	Lukas Czerner <lczerner@...hat.com>, Milan Broz <mbroz@...hat.com>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"achender\@linux.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

Jens Axboe <jaxboe@...ionio.com> writes:

> 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.

C'mon... it's bizarre!  Loopback mounting a block device, to make it
look like a block device?  Anyway...

> 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.

Fine by me.

Reviewed-by: Jeff Moyer <jmoyer@...hat.com>
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