Re: [PATCH 3/6] fs: distinguish between user initiated freeze and kernel initiated freeze

[Jan Kara]

On Tue 06-06-23 10:19:56, Darrick J. Wong wrote:
> On Thu, May 25, 2023 at 04:14:30PM +0200, Jan Kara wrote:
> > On Mon 22-05-23 16:42:00, Darrick J. Wong wrote:
> > > How about this as an alternative patch?  Kernel and userspace freeze
> > > state are stored in s_writers; each type cannot block the other (though
> > > you still can't have nested kernel or userspace freezes); and the freeze
> > > is maintained until /both/ freeze types are dropped.
> > > 
> > > AFAICT this should work for the two other usecases (quiescing pagefaults
> > > for fsdax pmem pre-removal; and freezing fses during suspend) besides
> > > online fsck for xfs.
> > > 
> > > --D
> > > 
> > > From: Darrick J. Wong <djwong@xxxxxxxxxx>
> > > Subject: fs: distinguish between user initiated freeze and kernel initiated freeze
> > > 
> > > Userspace can freeze a filesystem using the FIFREEZE ioctl or by
> > > suspending the block device; this state persists until userspace thaws
> > > the filesystem with the FITHAW ioctl or resuming the block device.
> > > Since commit 18e9e5104fcd ("Introduce freeze_super and thaw_super for
> > > the fsfreeze ioctl") we only allow the first freeze command to succeed.
> > > 
> > > The kernel may decide that it is necessary to freeze a filesystem for
> > > its own internal purposes, such as suspends in progress, filesystem fsck
> > > activities, or quiescing a device prior to removal.  Userspace thaw
> > > commands must never break a kernel freeze, and kernel thaw commands
> > > shouldn't undo userspace's freeze command.
> > > 
> > > Introduce a couple of freeze holder flags and wire it into the
> > > sb_writers state.  One kernel and one userspace freeze are allowed to
> > > coexist at the same time; the filesystem will not thaw until both are
> > > lifted.
> > > 
> > > Inspired-by: Luis Chamberlain <mcgrof@xxxxxxxxxx>
> > > Signed-off-by: Darrick J. Wong <djwong@xxxxxxxxxx>
> > 
> > Yes, this is exactly how I'd imagine it. Thanks for writing the patch!
> > 
> > I'd just note that this would need rebasing on top of Luis' patches 1 and
> > 2. Also:
> 
> I started doing that, but I noticed that after patch 1, freeze_super no
> longer leaves s_active elevated if the freeze is successful.  The
> callers drop the s_active ref that they themselves obtained, which
> means that we've now changed that behavior, right?  ioctl_fsfreeze now
> does:
> 
> 	if (!get_active_super(sb->s_bdev))
> 		return -ENOTTY;
> 
> (Increase ref)
> 
>         /* Freeze */
>         if (sb->s_op->freeze_super)
> 		ret = sb->s_op->freeze_super(sb);
> 	ret = freeze_super(sb);
> 
> (Not sure why we can do both here?)
> 
> 	deactivate_locked_super(sb);
> 
> (Decrease ref; net change to s_active is zero)
> 
> 	return ret;
> 
> Luis hasn't responded to my question, so I stopped.

Right. I kind of like how he's moved the locking out of freeze_super() /
thaw_super() but I agree this semantic change is problematic and needs much
more thought - e.g. with Luis' version the fs could be unmounted while
frozen which is going to spectacularly deadlock. So yeah let's just ignore
patch 1 for now.

Longer term I believe if the sb is frozen by userspace, we should refuse to
unmount it but that's a separate discussion for some other time.

> > BTW, when reading this code, I've spotted attached cleanup opportunity but
> > I'll queue that separately so that is JFYI.
> > 
> > > +#define FREEZE_HOLDER_USERSPACE	(1U << 1)	/* userspace froze fs */
> > > +#define FREEZE_HOLDER_KERNEL	(1U << 2)	/* kernel froze fs */
> > 
> > Why not start from 1U << 0? And bonus points for using BIT() macro :).
> 
> I didn't think filesystem code was supposed to be using stuff from
> vdso.h...

Hum, so BIT() macro is quite widely used in include/linux/ (from generic
headers e.g. in trace.h). include/linux/bits.h seems to be the right
include to use but I'm pretty sure include/linux/fs.h already gets this
header through something.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR