linux-kernel - Re: [PATCH] fs: Fix freeze_bdev()/thaw_bdev() accounting of bd_fsfreeze

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Date:   Fri, 8 Jan 2021 09:58:46 -0500 (EST)
From:   Bob Peterson <rpeterso@...hat.com>
To:     Satya Tangirala <satyat@...gle.com>
Cc:     Christoph Hellwig <hch@....de>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH] fs: Fix freeze_bdev()/thaw_bdev() accounting of
 bd_fsfreeze_sb

Hi,

> This is the bigger issue, and I'm not very familiar with this code either,
> so I'll defer to the experts. Yes, it's a change in behavior, but I think
> it makes sense to decrement the bd_fsfreeze_count in this case. Here's why:
> 
> If the blockdev is frozen by freeze_bdev while it's being unmounted, the
> bd_fsfreeze_count is incremented, but the freeze is ignored. Subsequent
> attempts to thaw the device will be ignored but return 0 because the sb
> is not found. When the device is mounted again, calls to freeze_bdev
> will bypass the call to freeze_super for the newly mounted sb, because
> bdev->bd_fsfreeze_count was then incremented from 1 to 2 in freeze_bdev.
> 
> 	if (++bdev->bd_fsfreeze_count > 1)
> 		goto done;
> 
> So you're freezing the device without really freezing the superblock.
> Seems like dangerous behavior to me. The new sb will only be frozen if
> a second thaw is done, which gets them back in sync. I suppose we could
> say this is acceptable loss, and your number of thaws should match your
> freezes, and if they don't: user error. Still, it seems like we should do
> something about it, like refuse to mount a frozen device. Perhaps it already
> does that; I'll need to do some research.

After some experiments, I've determined that my fears about the count are unfounded.
Consider my patch withdrawn. Sorry for the noise.

Bob Peterson