linux-kernel - Re: [PATCH 0/2] block: avoid to drop & re-add partitions if partitions aren't changed

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Message-ID: <20210218075703.GD284137@T590>
Date:   Thu, 18 Feb 2021 15:57:03 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, "Ewan D . Milne" <emilne@...hat.com>
Subject: Re: [PATCH 0/2] block: avoid to drop & re-add partitions if
 partitions aren't changed

On Wed, Feb 17, 2021 at 08:16:29AM +0100, Christoph Hellwig wrote:
> On Wed, Feb 17, 2021 at 11:07:14AM +0800, Ming Lei wrote:
> > Do you think it is correct for ioctl(BLKRRPART) to always drop/re-add
> > partition device node?
> 
> Yes, that is what it is designed to do.  The only reason to call this
> ioctl is when userspace software has written new partition table
> information to the disk.

I am wondering how userspace can know this design or implication since
this behavior wasn't documented anywhere.

For example, 'blockdev --rereadpt' can do it simply, without updating
partition table at all.

The reality is that almost of all the main userspace consumers of
ioctl(BLKRRPART) didn't follow such 'rule', then partitions node from
'bdev' fs can disappear & re-appear anytime. I believe it is one bug
from userspace view.

Thanks,
Ming