linux-kernel - Re: [PATCH/RFC] md: remove media-change code

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Date:   Thu, 14 Apr 2022 13:06:36 +0200
From:   Mariusz Tkaczyk <mariusz.tkaczyk@...ux.intel.com>
To:     "NeilBrown" <neilb@...e.de>
Cc:     Song Liu <song@...nel.org>, linux-raid@...r.kernel.org,
        linux-block@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Lennart Poettering <lennart@...ttering.net>
Subject: Re: [PATCH/RFC] md: remove media-change code

On Thu, 14 Apr 2022 16:06:05 +1000
"NeilBrown" <neilb@...e.de> wrote:

> md only ever used the media-change interfaces to trigger a partition
> rescan once the array became active.  Normally partition scan only
> happens when the disk is first added, and with md the disk is typically
> inactive when first added.
> 
> This rescan can now be achieved by simply setting GD_NEED_PART_SCAN.
> So do that, and remove all the rescan.
> 
Hi Neil,

I experimented in this area in the past, mainly on IMSM (external
metadata). My problem is described here:
https://lore.kernel.org/linux-raid/SA0PR11MB4542ECA84F72506B39C3C9F1FFEE0@SA0PR11MB4542.namprd11.prod.outlook.com/
I lost reproduction on newer kernels, probably changes in block layer hide
issue, it seems to be time race. The change you proposed could bring the issue
back.

The current way is working, so I consider your change as potentially dangerous.
Anyway, I will help with testing if Song decides to take it.

For external metadata we should impose partition read after mdmon start (when
md device becomes RW), so it should be synchronized with mdadm.
It could break autostart functionality for native metadata (if it is still
in use).

Eventually, external metadata should be handled separately.

Thanks,
Mariusz