linux-kernel - Re: [patch v3]DM: dm-insitu-comp: a compressed DM target for SSD

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Date:	Mon, 17 Mar 2014 16:00:40 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	Shaohua Li <shli@...nel.org>
Cc:	axboe@...nel.dk, dm-devel@...hat.com, linux-kernel@...r.kernel.org,
	agk@...hat.com
Subject: Re: [patch v3]DM: dm-insitu-comp: a compressed DM target for SSD

On Mon, Mar 17 2014 at  5:56am -0400,
Shaohua Li <shli@...nel.org> wrote:

> On Fri, Mar 14, 2014 at 06:44:45PM -0400, Mike Snitzer wrote:
> > On Fri, Mar 14 2014 at  5:40am -0400,
> > Shaohua Li <shli@...nel.org> wrote:
> > 
> > > On Mon, Mar 10, 2014 at 09:52:56AM -0400, Mike Snitzer wrote:
> > > > On Fri, Mar 07 2014 at  2:57am -0500,
> > > > Shaohua Li <shli@...nel.org> wrote:
> > > > 
> > > > > ping!
> > > > 
> > > > Hi,
> > > > 
> > > > I intend to get dm-insitu-comp reviewed for 3.15.  Sorry I haven't
> > > > gotten back with you before now, been busy tending to 3.14-rc issues.
> > > > 
> > > > I took a quick first pass over your code a couple weeks ago.  Looks to
> > > > be in great shape relative to coding conventions and the more DM
> > > > specific conventions.  Clearly demonstrates you have a good command of
> > > > DM concepts and quirks.
> > 
> > Think I need to eat my words from above at least partially.  Given you
> > haven't implemented any of the target suspend or resume hooks this
> > target will _not_ work properly across suspend + resume cycles that all
> > DM targets must support.
> > 
> > But we can obviously work through it with urgency for 3.15.
> > 
> > I've pulled your v3 patch into git and have overlayed edits from my
> > first pass.  Lots of funky wrapping to conform to 80 columns.  But
> > whitespace aside, I've added FIXME:s in the relevant files.  If you work
> > on any of these FIXMEs please send follow-up patches so that we don't
> > step on each others' toes.
> > 
> > Please see the 'for-3.15-insitu-comp' branch of this git repo:
> > git://git.kernel.org/pub/scm/linux/kernel/git/snitzer/linux.git
> 
> Thanks for your to look at it. I fixed them against your tree. Please check below patch.

I folded your changes in, and then committed a patch ontop that cleans
some code up.  But added 2 FIXMEs that still speak to pretty fundamental
problems with the architecture of the dm-insitu-comp target, see:
https://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/commit/?h=for-3.15-insitu-comp&id=8565ab6b04837591d03c94851c2f9f9162ce12f4

Unfortunately the single insitu_comp_wq workqueue that all insitu-comp
targets are to share isn't a workable solution.  Each target needs to
have resource isolation from other targets (imagine insitu-comp used for
multiple SSDs).  This is important for suspend too because you'll need
to flush/stop the workqueue.

You introduced a state machine for tracking suspending, suspended,
resumed.  This really isn't necessary.  During suspend you need to
flush_workqueue().  On resume you shouldn't need to do anything special.

As I noted in the commit, the thin and cache targets can serve as
references for how you can manage the workqueue across suspend/resume
and the lifetime of these workqueues relative to .ctr and .dtr.
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