linux-kernel - Re: [RFC v3 03/24] fs: distinguish between user initiated freeze and kernel initiated freeze

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Date:   Mon, 22 May 2023 17:33:05 -0700
From:   "Darrick J. Wong" <djwong@...nel.org>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     Jan Kara <jack@...e.cz>, hch@...radead.org, song@...nel.org,
        rafael@...nel.org, gregkh@...uxfoundation.org,
        viro@...iv.linux.org.uk, bvanassche@....org, ebiederm@...ssion.com,
        mchehab@...nel.org, keescook@...omium.org, p.raghav@...sung.com,
        linux-fsdevel@...r.kernel.org, kernel@...force.de,
        kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
        xfs <linux-xfs@...r.kernel.org>
Subject: Re: [RFC v3 03/24] fs: distinguish between user initiated freeze and
 kernel initiated freeze

On Sat, May 06, 2023 at 09:08:35PM -0700, Luis Chamberlain wrote:
> On Wed, Jan 18, 2023 at 10:28:12AM +0100, Jan Kara wrote:
> > On Tue 17-01-23 18:25:40, Darrick J. Wong wrote:
> > > [add linux-xfs to cc on this one]
> > > 
> > > On Fri, Jan 13, 2023 at 04:33:48PM -0800, Luis Chamberlain wrote:
> > > > Userspace can initiate a freeze call using ioctls. If the kernel decides
> > > > to freeze a filesystem later it must be able to distinguish if userspace
> > > > had initiated the freeze, so that it does not unfreeze it later
> > > > automatically on resume.
> > > 
> > > Hm.  Zooming out a bit here, I want to think about how kernel freezes
> > > should behave...
> > > 
> > > > Likewise if the kernel is initiating a freeze on its own it should *not*
> > > > fail to freeze a filesystem if a user had already frozen it on our behalf.
> > > 
> > > ...because kernel freezes can absorb an existing userspace freeze.  Does
> > > that mean that userspace should be prevented from undoing a kernel
> > > freeze?  Even in that absorption case?
> > > 
> > > Also, should we permit multiple kernel freezes of the same fs at the
> > > same time?  And if we do allow that, would they nest like freeze used to
> > > do?
> > > 
> > > (My suggestions here are 'yes', 'yes', and '**** no'.)
> > 
> > Yeah, makes sense to me. So I think the mental model to make things safe
> > is that there are two flags - frozen_by_user, frozen_by_kernel - and the
> > superblock is kept frozen as long as either of these is set.
> 
> Makes sense to me.

Just sent a patch for this, sorry it took a couple of weeks while I was
busy merging in parent pointers...

--D

>   Luis