linux-kernel - Re: [PATCH 6/6] fs: add automatic kernel fs freeze / thaw and remove kthread freezing

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Date:   Tue, 16 May 2023 08:17:18 -0700
From:   "Darrick J. Wong" <djwong@...nel.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     Luis Chamberlain <mcgrof@...nel.org>, hch@...radead.org,
        sandeen@...deen.net, song@...nel.org, rafael@...nel.org,
        gregkh@...uxfoundation.org, viro@...iv.linux.org.uk, jack@...e.cz,
        jikos@...nel.org, bvanassche@....org, ebiederm@...ssion.com,
        mchehab@...nel.org, keescook@...omium.org, p.raghav@...sung.com,
        da.gomez@...sung.com, linux-fsdevel@...r.kernel.org,
        kernel@...force.de, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/6] fs: add automatic kernel fs freeze / thaw and remove
 kthread freezing

On Tue, May 09, 2023 at 11:20:13AM +1000, Dave Chinner wrote:
> On Sun, May 07, 2023 at 06:17:17PM -0700, Luis Chamberlain wrote:
> > Add support to automatically handle freezing and thawing filesystems
> > during the kernel's suspend/resume cycle.
> > 
> > This is needed so that we properly really stop IO in flight without
> > races after userspace has been frozen. Without this we rely on
> > kthread freezing and its semantics are loose and error prone.
> > For instance, even though a kthread may use try_to_freeze() and end
> > up being frozen we have no way of being sure that everything that
> > has been spawned asynchronously from it (such as timers) have also
> > been stopped as well.
> > 
> > A long term advantage of also adding filesystem freeze / thawing
> > supporting during suspend / hibernation is that long term we may
> > be able to eventually drop the kernel's thread freezing completely
> > as it was originally added to stop disk IO in flight as we hibernate
> > or suspend.
> > 
> > This does not remove the superfluous freezer calls on all filesystems.
> > Each filesystem must remove all the kthread freezer stuff and peg
> > the fs_type flags as supporting auto-freezing with the FS_AUTOFREEZE
> > flag.
> > 
> > Subsequent patches remove the kthread freezer usage from each
> > filesystem, one at a time to make all this work bisectable.
> > Once all filesystems remove the usage of the kthread freezer we
> > can remove the FS_AUTOFREEZE flag.
> > 
> > Reviewed-by: Jan Kara <jack@...e.cz>
> > Signed-off-by: Luis Chamberlain <mcgrof@...nel.org>
> > ---
> >  fs/super.c             | 50 ++++++++++++++++++++++++++++++++++++++++++
> >  include/linux/fs.h     | 14 ++++++++++++
> >  kernel/power/process.c | 15 ++++++++++++-
> >  3 files changed, 78 insertions(+), 1 deletion(-)
> 
> .....
> 
> > diff --git a/kernel/power/process.c b/kernel/power/process.c
> > index cae81a87cc91..7ca7688f0b5d 100644
> > --- a/kernel/power/process.c
> > +++ b/kernel/power/process.c
> > @@ -140,6 +140,16 @@ int freeze_processes(void)
> >  
> >  	BUG_ON(in_atomic());
> >  
> > +	pr_info("Freezing filesystems ... ");
> > +	error = iterate_supers_reverse_excl(fs_suspend_freeze_sb, NULL);
> > +	if (error) {
> > +		pr_cont("failed\n");
> > +		iterate_supers_excl(fs_suspend_thaw_sb, NULL);
> > +		thaw_processes();
> > +		return error;
> 
> That looks wrong. i.e. if the sb iteration fails to freeze a
> filesystem (for whatever reason) then every userspace frozen
> filesystem will be thawed by this call, right? i.e. it will thaw
> more than just the filesystems frozen by the suspend freeze
> iteration before it failed.
> 
> Don't we only want to thaw the superblocks we froze before the
> failure occurred? i.e. the "undo" iteration needs to start from the
> last superblock we successfully froze and then only walk to the tail
> of the list we started from?

I think fs_suspend_thaw_sb calls thaw_super(..., false), which will not
undo a userspace freeze.  So strictly speaking the answer to your
question is (AFAICT) "no it won't"

That said, I read this and also had a raised-eyebrow moment -- we
shouldn't (un?)touch superblocks that fs_suspend_freeze_sb didn't touch
in the first place.

I wonder if we should be using that NULL parameter to keep track of the
last super that fs_suspend_freeze_sb didn't fail on?

--D

> > +	}
> > +	pr_cont("done.\n");
> > +
> >  	/*
> >  	 * Now that the whole userspace is frozen we need to disable
> >  	 * the OOM killer to disallow any further interference with
> > @@ -149,8 +159,10 @@ int freeze_processes(void)
> >  	if (!error && !oom_killer_disable(msecs_to_jiffies(freeze_timeout_msecs)))
> >  		error = -EBUSY;
> >  
> > -	if (error)
> > +	if (error) {
> > +		iterate_supers_excl(fs_suspend_thaw_sb, NULL);
> >  		thaw_processes();
> > +	}
> 
> Does this also have the same problem? i.e. if fs_suspend_freeze_sb()
> skips over superblocks that are already userspace frozen without any
> error, then this will incorrectly thaw those userspace frozen
> filesystems.
> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@...morbit.com