linux-kernel - Re: [PATCH 3/4] Staging: zram: allow partial page operations

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Date:	Tue, 14 Jun 2011 10:49:26 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Jerome Marchand <jmarchan@...hat.com>
Cc:	Nitin Gupta <ngupta@...are.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Linux Kernel List <linux-kernel@...r.kernel.org>,
	Robert Jennings <rcj@...ux.vnet.ibm.com>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [PATCH 3/4] Staging: zram: allow partial page operations

Jerome Marchand <jmarchan@...hat.com> writes:

> On 06/10/2011 06:41 PM, Nitin Gupta wrote:
>> On 06/10/2011 06:28 AM, Jerome Marchand wrote:
>>> Commit 7b19b8d45b216ff3186f066b31937bdbde066f08 (zram: Prevent overflow
>>> in logical block size) introduced ZRAM_LOGICAL_BLOCK_SIZE constant to
>>> prevent overflow of logical block size on 64k page kernel.
>>> However, the current implementation of zram only allow operation on block
>>> of the same size as a page. That makes theorically legit 4k requests fail
>>> on 64k page kernel.
>>>
>>> This patch makes zram allow operation on partial pages. Basically, it
>>> means we still do operations on full pages internally, but only copy the
>>> relevent segments from/to the user memory.
>>>
>> 
>> Couldn't we just change struct queue_limits.logical_block_size type to 
>> unsigned int or something so it could hold value of 64K? Then we could
>> avoid making all these changes to handle partial page requests.
>> 
>> Thanks,
>> Nitin
>
> I believe logical_block_size is meant to be small. I don't know if it is
> reasonable to set it to such a big value as 64k. I CCed Jens and Martin to
> have a more valuable opinion on the matter.

I don't think there's any reason the logical block size can't be
increased.  For zram, so long as you don't care that the minimum I/O
size is 64k on these systems (and by you, I mean the users of zram, like
file systems, or anything using the block device directly), then it's
a fine trade-off to make.

Jens, Martin, what do you guys think about bumping the size of the
queue_limits.logical_block_size?

Cheers,
Jeff
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