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

[Jan Kara]

On Wed 07-06-23 09:31:10, 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:
> > 
> > > +	if (sbw->frozen == SB_FREEZE_COMPLETE) {
> > > +		switch (who) {
> > > +		case FREEZE_HOLDER_KERNEL:
> > > +			if (sbw->freeze_holders & FREEZE_HOLDER_KERNEL) {
> > > +				/*
> > > +				 * Kernel freeze already in effect; caller can
> > > +				 * try again.
> > > +				 */
> > > +				deactivate_locked_super(sb);
> > > +				return -EBUSY;
> > > +			}
> > > +			if (sbw->freeze_holders & FREEZE_HOLDER_USERSPACE) {
> > > +				/*
> > > +				 * Share the freeze state with the userspace
> > > +				 * freeze already in effect.
> > > +				 */
> > > +				sbw->freeze_holders |= who;
> > > +				deactivate_locked_super(sb);
> > > +				return 0;
> > > +			}
> > > +			break;
> > > +		case FREEZE_HOLDER_USERSPACE:
> > > +			if (sbw->freeze_holders & FREEZE_HOLDER_USERSPACE) {
> > > +				/*
> > > +				 * Userspace freeze already in effect; tell
> > > +				 * the caller we're busy.
> > > +				 */
> > > +				deactivate_locked_super(sb);
> > > +				return -EBUSY;
> > > +			}
> > > +			if (sbw->freeze_holders & FREEZE_HOLDER_KERNEL) {
> > > +				/*
> > > +				 * Share the freeze state with the kernel
> > > +				 * freeze already in effect.
> > > +				 */
> > > +				sbw->freeze_holders |= who;
> > > +				deactivate_locked_super(sb);
> > > +				return 0;
> > > +			}
> > > +			break;
> > > +		default:
> > > +			BUG();
> > > +			deactivate_locked_super(sb);
> > > +			return -EINVAL;
> > > +		}
> > > +	}
> > 
> > Can't this be simplified to:
> > 
> > 	BUG_ON(who & ~(FREEZE_HOLDER_USERSPACE | FREEZE_HOLDER_KERNEL));
> > 	BUG_ON(!(!(who & FREEZE_HOLDER_USERSPACE) ^
> > 	       !(who & FREEZE_HOLDER_KERNEL)));
> > retry:
> > 	if (sb->s_writers.freeze_holders & who)
> > 		return -EBUSY;
> > 	/* Already frozen by someone else? */
> > 	if (sb->s_writers.freeze_holders & ~who) {
> > 		sb->s_writers.freeze_holders |= who;
> > 		return 0;
> > 	}
> > 
> > Now the only remaining issue with the code is that the two different
> > holders can be attempting to freeze the filesystem at once and in that case
> > one of them has to wait for the other one instead of returning -EBUSY as
> > would happen currently. This can happen because we temporarily drop
> > s_umount in freeze_super() due to lock ordering issues. I think we could
> > do something like:
> > 
> > 	if (!sb_unfrozen(sb)) {
> > 		up_write(&sb->s_umount);
> > 		wait_var_event(&sb->s_writers.frozen,
> > 			       sb_unfrozen(sb) || sb_frozen(sb));
> > 		down_write(&sb->s_umount);
> > 		goto retry;
> > 	}
> > 
> > and then sprinkle wake_up_var(&sb->s_writers.frozen) at appropriate places
> > in freeze_super().
> 
> If we implemented this behavior change, it ought to be a separate patch.
> 
> For the case where the kernel is freezing the fs and userspace wants to
> start freezing the fs, we could make userspace wait and then share the
> kernel freeze.

Yes.

> For any case where the fs is !unfrozen and the kernel wants to start
> freezing the fs, I think I'd rather return EBUSY immediately and let the
> caller decide to wait and/or call back.

Possibly, although I thought that if userspace has frozen the fs and kernel
wants to freeze, we want to return success? At least that was what I think
your patches were doing. And then I don't see the point why we should be
returning EBUSY if userspace is in the middle of the freeze. So what's the
intended semantics?
 
> For the case where one userspace thread is freezing the fs and another
> userspace thread wants to start freezing the fs, I think the current
> behavior of returning EBUSY immediately is ok.

Yes.

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