On Tue 17-04-18 17:59:36, Luis R. Rodriguez wrote: > On Thu, Dec 21, 2017 at 12:03:29PM +0100, Jan Kara wrote: > > On Fri 01-12-17 22:13:27, Luis R. Rodriguez wrote: > > > > > > I'll note that its still not perfectly clear if really the semantics behind > > > freeze_bdev() match what I described above fully. That still needs to be > > > vetted for. For instance, does thaw_bdev() keep a superblock frozen if we > > > an ioctl initiated freeze had occurred before? If so then great. Otherwise > > > I think we'll need to distinguish the ioctl interface. Worst possible case > > > is that bdev semantics and in-kernel semantics differ somehow, then that > > > will really create a holy fucking mess. > > > > I believe nobody really thought about mixing those two interfaces to fs > > freezing and so the behavior is basically defined by the implementation. > > That is: > > > > freeze_bdev() on sb frozen by ioctl_fsfreeze() -> EBUSY > > Note below as well on your *future* freeze_super() implementation. > > > freeze_bdev() on sb frozen by freeze_bdev() -> success > > ioctl_fsfreeze() on sb frozen by freeze_bdev() -> EBUSY > > ioctl_fsfreeze() on sb frozen by ioctl_fsfreeze() -> EBUSY > > > > thaw_bdev() on sb frozen by ioctl_fsfreeze() -> EINVAL > > Phew, so this is what we want for the in-kernel freezing so we're good > and *can* combine these then. > > > ioctl_fsthaw() on sb frozen by freeze_bdev() -> success > > > > What I propose is the following API: > > > > freeze_super_excl() > > - freezes superblock, returns EBUSY if the superblock is already frozen > > (either by another freeze_super_excl() or by freeze_super()) > > freeze_super() > > - this function will make sure superblock is frozen when the function > > returns with success. > > That's straight forward. > > > It can be nested with other freeze_super() or > > freeze_super_excl() calls > > This is where it can get hairy. More below. > > > (this second part is different from how > > freeze_bdev() behaves currently but AFAICT this behavior is actually > > what all current users of freeze_bdev() really want - just make sure > > fs cannot be written to) > > If we can agree to this, then sure. However there are two types of > possible nested calls to consider, one where the sb was already frozen > by an IOCTL, and the other where it was initiated by either another > freeze_super_excl() or another freeze_super() call which is currently > being processed. For the first type, its easy to say the device is > already frozen as such return success. If the freezing is ongoing, > we may want to wait or not wait, and this will depend on our current > use cases for freeze_bdev(). A side note since I'm not sure I wrote this down in my previous email: I want ioctl_fsfreeze() directly use freeze_super_excl(). Now to your freeze in progress question: freeze_super_excl() can immediately return EBUSY when there's freezing in progress. OTOH freeze_super() always has to wait for the current freeze / thaw to finish and then do what's necessary. I don't see a use case where you'd like to have freeze_super() not wait. > As you noted above, freeze_bdev() currently returns EBUSY if we had > the sb already frozen by ioctl_fsfreeze(). It may be a welcomed > enhancement to correct the semantics first to address the first case, > but keep the EBUSY for the other case. A secondary patch could then > add a completion mechanism and let callers decide to either wait or not. > *Iff* the caller did not opt-in to wait we keep the EBUSY return. You're now speaking about steps to transition to the new API, right? I'd structure the transition as follows: 1) Move bdev->bd_fsfreeze_count to a superblock. 2) Make freeze_super() grab the counter as well, thaw_super() drops it and unfreezes the filesystem only if the counter dropped to zero. 3) Rename freeze_super() to freeze_super_excl(). 4) Only now I'd go for messing with freeze_bdev() as it now combines sanely with freeze_super_excl(). Probably I'd just implement new freeze_super() with the desired semantics (including waiting for ongoing operation to finish). 5) And then switch all users (there are 4 in the kernel) from freeze_bdev() to freeze_super() with the justification in each case why the new semantics is actually desirable. 6) Drop old freeze_bdev() - note that only one freeze_bdev() user (in drivers/md/dm.c) is actually interested in passing bdev, all the others are better off just passing in superblock to new freeze_super(). Anyway for that user in dm we might still provide a convenience wrapper to grab the superblock and call new freeze_super() on it. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
