linux-kernel - Re: [PATCH 3/5] dm: support retrieving struct dm_target from struct dm

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Date: Fri, 17 May 2024 10:33:10 -0400
From: Benjamin Marzinski <bmarzins@...hat.com>
To: YangYang <yang.yang@...o.com>
Cc: Alasdair Kergon <agk@...hat.com>, Mike Snitzer <snitzer@...nel.org>,
        Mikulas Patocka <mpatocka@...hat.com>, dm-devel@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] dm: support retrieving struct dm_target from struct
 dm_dev

On Fri, May 17, 2024 at 03:48:49PM +0800, YangYang wrote:
> On 2024/5/16 23:29, Benjamin Marzinski wrote:
> > On Thu, May 16, 2024 at 09:55:53AM +0800, YangYang wrote:
> > > On 2024/5/15 23:42, Benjamin Marzinski wrote:
> > > > On Tue, May 14, 2024 at 05:04:42PM +0800, Yang Yang wrote:

> > > 
> > > If I understand correctly, you are suggesting to iterate through all the
> > > targets, handling those with sends_pass_around_flush set, and skipping
> > > those where sends_pass_around_flush is not set. I believe this approach
> > > may result in some CPU wastage.
> > > 
> > >    for i in {0..1023}; do
> > >      echo $((8000*$i)) 8000 linear /dev/sda2 $((16384*$i))
> > >    done | sudo dmsetup create example
> > > 
> > > In this specific scenario, a single iteration of the loop is all that
> > > is needed.
> > 
> > It's just one iteration of the loop either way. You either loop through
> > the targets or the devices.  It's true that if you have lots of targets
> > all mapped to the same device, you would waste time looping through all
> > the targets instead of looping through the devices.  But if you only had
> > one striped target mapped to lots of devices, you would waste time
> > looping through all of the devices instead of looping through the
> > targets.
> 
> Yes, I get your point. This patchset may make things even worse for
> the striped target.
> I am just curious, in what scenario is the "dm-strip" target mapped to
> a large number of underlying devices from the same block device.
> 

I don't think anyone in the real world does create dm-stripe devices with a
huge number of stripe table devices. My point was that it didn't seem
obvious me that looping through the targets was a significant problem
compared to looping through the devices.

At any rate, Mikulas's patch already does this optimally, even for
targets like dm-stripe, so it doesn't really matter now.

-Ben