linux-kernel - Re: [dm-devel] [PATCH] dm: remake of the verity target

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Message-ID: <20120323031532.GP27051@google.com>
Date:	Thu, 22 Mar 2012 20:15:32 -0700
From:	Mandeep Singh Baines <msb@...omium.org>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	Mandeep Singh Baines <msb@...omium.org>,
	device-mapper development <dm-devel@...hat.com>,
	Steffen Klassert <steffen.klassert@...unet.com>,
	Will Drewry <wad@...omium.org>, linux-kernel@...r.kernel.org,
	Elly Jones <ellyjones@...omium.org>,
	Olof Johansson <olofj@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alasdair G Kergon <agk@...hat.com>,
	Milan Broz <mbroz@...hat.com>, taysom@...omium.org
Subject: Re: [dm-devel] [PATCH] dm: remake of the verity target

Mikulas Patocka (mpatocka@...hat.com) wrote:
> 
> 
> On Thu, 22 Mar 2012, Mandeep Singh Baines wrote:
> 
> > Mikulas Patocka (mpatocka@...hat.com) wrote:
> > > 
> > > This is improved patch that supports both the old format and the new 
> > > format. I checked that it is interoperable with with the old Google 
> > > userspace tool and with the original Google kernel driver.
> > > 
> > 
> > Thanks much for doing this:)
> > 
> > This looks good but would a prepend/append flag be better?
> 
> It does more than changing prepend/append salt. I changed alignment in the 
> new format so that it doesn't have to use a multiply instruction.
> 
> In the old format, if digest size is not a power of two, all digests are 
> placed first and the rest of the block is padded with zeros. In the new 
> format, each digest is padded with zeros to a power of two.
> 
> For example, when using sha1, the old format padding looks like this:
> aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbbbbbb
> bbbbbbbbbbbbbbbbccccccccccccccccccccccccccccccccccccccccdddddddd
> dddddddddddddddddddddddddddddddd00000000000000000000000000000000
> 0000000000000000000000000000000000000000000000000000000000000000
> 
> ... and the new format padding looks like this:
> aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa000000000000000000000000
> bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb000000000000000000000000
> cccccccccccccccccccccccccccccccccccccccc000000000000000000000000
> dddddddddddddddddddddddddddddddddddddddd000000000000000000000000
> 
> The version "0" (first argument in the target line) actually means the old 
> style padding and the salt is hashed after the data. The version "1" means 
> new style padding and the salt is hashed before the data. If someone comes 
> with another format, we can use version "2" for it, etc.
> 

+cc taysom

Makes sense.

Signed-off-by: Mandeep Singh Baines <msb@...omium.org>

Speaking of V2, one idea a colleague of mine (taysom) just had was to
drop the power of 2 alignment. For SHA-1, this shrinks the tree by 37.5 %.
You have to replace the shifts with divides but the reduction in I/O
more than makes up. For the different levels, you could pre-calculate
the divisor.

Regards,
Mandeep

> Mikulas
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