linux-kernel - RE: [PATCH 1/2] Tools: hv: vss: Skip freezing filesystems backed by loop

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Date:   Mon, 17 Jul 2017 18:59:32 +0000
From:   "Alex Ng (LIS)" <alexng@...rosoft.com>
To:     Greg KH <gregkh@...uxfoundation.org>,
        KY Srinivasan <kys@...rosoft.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
        "olaf@...fle.de" <olaf@...fle.de>,
        "apw@...onical.com" <apw@...onical.com>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "leann.ogasawara@...onical.com" <leann.ogasawara@...onical.com>,
        "marcelo.cerri@...onical.com" <marcelo.cerri@...onical.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Vyronas Tsingaras <vyronas@...ingaras.me>
Subject: RE: [PATCH 1/2] Tools: hv: vss: Skip freezing filesystems backed by
 loop

My apologies. The changelog comment should be more descriptive.

Since a loop device is backed by a file, a backup will already result in
its parent filesystem being frozen. It's sufficient to just freeze the
parent filesystem, so we can skip the loop device.

This avoids a situation where a loop device and its parent filesystem are
both frozen and then thawed out of order. For example, if the loop device
is enumerated first, we would thaw it while its parent filesystem is still
frozen. The thaw operation fails and the loop device remains frozen.

I'll work with KY to send an updated patch with a better comment.

> -----Original Message-----
> From: Greg KH [mailto:gregkh@...uxfoundation.org]
> Sent: Monday, July 17, 2017 6:00 AM
> To: KY Srinivasan <kys@...rosoft.com>
> Cc: linux-kernel@...r.kernel.org; devel@...uxdriverproject.org;
> olaf@...fle.de; apw@...onical.com; vkuznets@...hat.com;
> jasowang@...hat.com; leann.ogasawara@...onical.com;
> marcelo.cerri@...onical.com; Stephen Hemminger
> <sthemmin@...rosoft.com>; Alex Ng (LIS) <alexng@...rosoft.com>;
> Vyronas Tsingaras <vyronas@...ingaras.me>
> Subject: Re: [PATCH 1/2] Tools: hv: vss: Skip freezing filesystems backed by
> loop
> 
> On Sun, Jun 25, 2017 at 12:47:45PM -0700, kys@...hange.microsoft.com
> wrote:
> > From: Alex Ng <alexng@...sages.microsoft.com>
> >
> > Skip loop devices from the freeze/thaw operation.
> 
> Ok, but why?  What is the consequences of this?  Please provide a much
> better changelog comment, especially for something that is supposed to
> be a "fix".
> 
> thanks,
> 
> greg k-h