Re: [dm-devel] [PATCH v2] dm init: add dm-mod.waitfor to wait for asynchronously probed block devices

From: Peter Korsgaard <peter@korsgaard.com>
To: Mike Snitzer <snitzer@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>,
	Mike Snitzer <snitzer@kernel.org>,
	dm-devel@redhat.com, fabio.aiuto@amarulasolutions.com,
	Helen Koike <helen.koike@collabora.com>,
	michael@amarulasolutions.com
Subject: Re: [dm-devel] [PATCH v2] dm init: add dm-mod.waitfor to wait for asynchronously probed block devices
Date: Thu, 01 Dec 2022 08:28:05 +0100	[thread overview]
Message-ID: <87bkond1mi.fsf@dell.be.48ers.dk> (raw)
In-Reply-To: <Y4fiQvg9OOATD5Cv@redhat.com> (Mike Snitzer's message of "Wed, 30 Nov 2022 18:07:46 -0500")

>>>>> "Mike" == Mike Snitzer <snitzer@redhat.com> writes:

Hi,

 > On Thu, Nov 24 2022 at  5:35P -0500,
 > Peter Korsgaard <peter@korsgaard.com> wrote:

 >> > +	if (waitfor[0])
 >> > +		DMINFO("all devices available");
 >> > +

 > Why 20?  Also, why is waiting indefinitely OK?  Would really like to
 > hear from other consumers of dm-init that this module param is useful
 > and needed.

Mainly because of checkpatch.pl ;) prepare_namespace() uses msleep(5)
but checkpatch complains about it, so I made it 20 instead. It doesn't
really matter much, as long as it is small enough to not delay boots too
much (E.G. this is typically used for embedded system where boot time is
critical).

I would say that it DOES make sense to wait forever, similar to how it
is done in prepare_namespace() when rootwait is passed. This waitfor
bootargs is used for root-on-dm, so we cannot continue until the
underlying devices are available.

As mentioned,
https://lore.kernel.org/all/20220406154631.277107-1-fabio.aiuto@amarulasolutions.com/
is an alternatively (less nice) approach to solve the same issue, so yes
- It is useful and needed.

-- 
Bye, Peter Korsgaard

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