linux-kernel - Re: [PATCH] dm: add module parameter bdev

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Date:   Tue, 26 Oct 2021 23:55:43 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Mark-PK Tsai <mark-pk.tsai@...iatek.com>
Cc:     agk@...hat.com, snitzer@...hat.com, dm-devel@...hat.com,
        matthias.bgg@...il.com, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, tequila.yuang@...iatek.com,
        yj.chiang@...iatek.com
Subject: Re: [PATCH] dm: add module parameter bdev_wait

On Fri, Oct 22, 2021 at 02:47:47PM +0800, Mark-PK Tsai wrote:
> dm_early_create() fail if the target block device not found in
> the late init stage.
> The block device is created in mmc_rescan() which kernel do
> it async by queue it into system_freezable_wq.
> Add module param bdev_wait to support waiting the block device
> ready before dm_early_create() like prepare_namespace() does.

This adds a busy loop.  There is a reason why we usually set up
stacking drivers from userspace, in that we have a working event
system there.  So the answer is not to add further hacks to the
in-kernel DM setup hack, but to stop using it.