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From:   <yael.chemla@foss.arm.com>
To:     "'Mike Snitzer'" <snitzer@redhat.com>
Cc:     "'Alasdair Kergon'" <agk@redhat.com>, <dm-devel@redhat.com>,
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        "'Yael Chemla'" <yael.chemla@arm.com>,
        "'Eric Biggers'" <ebiggers3@gmail.com>
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Subject: RE: [PATCH 2/2] md: dm-verity: allow parallel processing of bio blocks
Date:   Tue, 27 Mar 2018 11:55:10 +0300
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Hi Mike
I need to rewrite these patches according to issues you and Eric Biggers mentioned.
please drop this v1 patch.
Thank you,
Yael

-----Original Message-----
From: Mike Snitzer <snitzer@redhat.com> 
Sent: Tuesday, 27 March 2018 4:07
To: Yael Chemla <yael.chemla@foss.arm.com>
Cc: Alasdair Kergon <agk@redhat.com>; dm-devel@redhat.com; linux-kernel@vger.kernel.org; ofir.drang@gmail.com; Yael Chemla <yael.chemla@arm.com>
Subject: Re: [PATCH 2/2] md: dm-verity: allow parallel processing of bio blocks

On Sun, Mar 25 2018 at  2:41pm -0400,
Yael Chemla <yael.chemla@foss.arm.com> wrote:

>  Allow parallel processing of bio blocks by moving to async. 
> completion  handling. This allows for better resource utilization of 
> both HW and  software based hash tfm and therefore better performance 
> in many cases,  depending on the specific tfm in use.
>  
>  Tested on ARM32 (zynq board) and ARM64 (Juno board).
>  Time of cat command was measured on a filesystem with various file sizes.
>  12% performance improvement when HW based hash was used (ccree driver).
>  SW based hash showed less than 1% improvement.
>  CPU utilization when HW based hash was used presented 10% less 
> context  switch, 4% less cycles and 7% less instructions. No 
> difference in  CPU utilization noticed with SW based hash.
>  
> Signed-off-by: Yael Chemla <yael.chemla@foss.arm.com>

This one had various issues.  I've fixed most of what I saw and staged in linux-next (purely for build test coverage purposes).  I may drop this patch if others disagree with it (or my sg deallocation in the error path question isn't answered).

I've staged the changes here (and in linux-next via 'for-next'):
https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/log/?h=dm-4.17

I switched all the new GFP_KERNEL uses to GFP_NOIO.  The fact that you're doing allocations at all (per IO) is bad enough.  Using GFP_KERNEL is a serious liability (risk of deadlock if dm-verity were to be used for something like.. swap.. weird setup but possible).

But the gfp flags aside, the need for additional memory and the expectation of scalable async parallel IO is potentially at odds with changes like this (that I just staged, and had to rebase your 2 patches ontop of):
https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.17&id=a89f6a2cfec86fba7a115642ff082cb4e9450ea6

So I'm particulalry interested to hear from google folks to understand if they are OK with your proposed verity async crypto API use.

Mike