linux-kernel - Re: [PATCH 1/2] block: fix leaks associated with discard request payload

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Date:	Wed, 30 Jun 2010 13:57:28 +0300
From:	Boaz Harrosh <bharrosh@...asas.com>
To:	Christoph Hellwig <hch@....de>
CC:	James Bottomley <James.Bottomley@...e.de>,
	Mike Snitzer <snitzer@...hat.com>, axboe@...nel.dk,
	dm-devel@...hat.com, linux-kernel@...r.kernel.org,
	martin.petersen@...cle.com, akpm@...ux-foundation.org,
	linux-scsi@...r.kernel.org,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Subject: Re: [PATCH 1/2] block: fix leaks associated with discard request
 payload

On 06/30/2010 01:41 PM, Christoph Hellwig wrote:
> On Wed, Jun 30, 2010 at 01:25:01PM +0300, Boaz Harrosh wrote:
>> OK, Thanks, I see. Is it one of these operations, (like we have in OSD) where
>> the CDB information spills into the payload? like the scatter-gather and extent
>> lists and such.
> 
> For UNMAP the payload is a list of block number / length pairs, while
> the CDB itself doesn't contain any information like that.  It's a rather
> awkward command.
> 

How big can that be? could we, maybe, use the sense_buffer, properly allocated
already?

>> Do we actually use a WRITE_SAME which is not zero? for what use?
> 
> The kernel doesn't issue any WRITE SAME without the unmap bit set.

So if the unmap bit is set then the page can just be zero, right?

I still think a static zero-page is a worth while optimization. And
block-drivers can take care with special needs with a private mem_pool
or something. For the discard-type user and generic block layer the
page is just an implementation specific residue, No?

But don't mind me, I'm just babbling. Not that I'll do anything about it.
Boaz
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