On Wed, Jun 07, 2023 at 10:46:10PM +0200, Jan Kara wrote: > 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 <djw...@kernel.org> > > > > 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 <mcg...@kernel.org> > > > > Signed-off-by: Darrick J. Wong <djw...@kernel.org> > > > > > > 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?
Yes. Apologies, I tripped over the four-states-of-gray thing and forgot that frozen != !unfrozen. > 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? Let me try again: "For the case where one kernel thread is freezing the fs and another kernel thread wants to start freezing the fs, return -EBUSY immediately. "For the case where userspace is freezing the fs and kernel wants to start freezing the fs, return -EBUSY immediately. Callers decide if they want to sleep and/or retry the operation." --D > > 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 <j...@suse.com> > SUSE Labs, CR _______________________________________________ kexec mailing list kexec@lists.infradead.org http://lists.infradead.org/mailman/listinfo/kexec