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From: Jan Kara <jack@suse.cz>
To: "Luis R. Rodriguez" <mcgrof@kernel.org>
Cc: Jan Kara <jack@suse.cz>,
	viro@zeniv.linux.org.uk, bart.vanassche@wdc.com,
	ming.lei@redhat.com, tytso@mit.edu, darrick.wong@oracle.com,
	jikos@kernel.org, rjw@rjwysocki.net, pavel@ucw.cz,
	len.brown@intel.com, linux-fsdevel@vger.kernel.org,
	boris.ostrovsky@oracle.com, jgross@suse.com,
	todd.e.brandt@linux.intel.com, nborisov@suse.com,
	martin.petersen@oracle.com, ONeukum@suse.com,
	oleksandr@natalenko.name, oleg.b.antonyan@gmail.com,
	yu.chen.surf@gmail.com, dan.j.williams@intel.com,
	linux-pm@vger.kernel.org, linux-block@vger.kernel.org,
	linux-xfs@vger.kernel.org, linux-kernel@vger.kernel.org,
	jlayton@redhat.com
Subject: Re: [PATCH 03/11] fs: add frozen sb state helpers
Date: Wed, 18 Apr 2018 12:12:21 +0200
Message-ID: <20180418101221.nzkpkuoojruuiqoc@quack2.suse.cz> (raw)
In-Reply-To: <20180418005936.j3fa5rogfm353clf@garbanzo.do-not-panic.com>

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@suse.com>
SUSE Labs, CR

  reply index

Thread overview: 47+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-11-29 23:23 [PATCH 00/11] fs: use freeze_fs on suspend/hibernate Luis R. Rodriguez
2017-11-29 23:23 ` [PATCH 01/11] fs: provide unlocked helper for freeze_super() Luis R. Rodriguez
2017-11-30 16:58   ` Jan Kara
2017-11-29 23:23 ` [PATCH 02/11] fs: provide unlocked helper thaw_super() Luis R. Rodriguez
2017-11-30 16:59   ` Jan Kara
2017-11-29 23:23 ` [PATCH 03/11] fs: add frozen sb state helpers Luis R. Rodriguez
2017-11-30 17:13   ` Jan Kara
2017-11-30 19:05     ` Luis R. Rodriguez
2017-12-01 11:47       ` Jan Kara
2017-12-01 21:13         ` Luis R. Rodriguez
2017-12-21 11:03           ` Jan Kara
2018-04-18  0:59             ` Luis R. Rodriguez
2018-04-18 10:12               ` Jan Kara [this message]
2018-04-20 18:49               ` Luis R. Rodriguez
2018-04-21 23:53                 ` Jan Kara
2018-04-22  1:22                   ` Luis R. Rodriguez
2018-04-22  2:53     ` Luis R. Rodriguez
2017-11-29 23:23 ` [PATCH 04/11] fs: distinguish between user initiated freeze and kernel initiated freeze Luis R. Rodriguez
2017-11-29 23:23 ` [PATCH 05/11] fs: add iterate_supers_excl() and iterate_supers_reverse_excl() Luis R. Rodriguez
2017-11-29 23:48   ` Rafael J. Wysocki
2017-11-30  0:22     ` Luis R. Rodriguez
2017-11-30  1:34     ` Dave Chinner
2017-11-30  1:40       ` Rafael J. Wysocki
2017-11-30 16:57   ` Jan Kara
2017-11-29 23:23 ` [PATCH 06/11] fs: freeze on suspend and thaw on resume Luis R. Rodriguez
2017-11-29 23:23 ` [PATCH 07/11] xfs: remove not needed freezing calls Luis R. Rodriguez
2017-11-30 16:21   ` Jan Kara
2017-11-30 20:32     ` Rafael J. Wysocki
2017-11-30 23:30       ` Dave Chinner
2017-11-30 23:40         ` Rafael J. Wysocki
2017-11-29 23:23 ` [PATCH 08/11] ext4: " Luis R. Rodriguez
2017-11-29 23:23 ` [PATCH 09/11] f2fs: " Luis R. Rodriguez
2017-11-29 23:23 ` [PATCH 10/11] nilfs2: " Luis R. Rodriguez
2017-11-29 23:23 ` [PATCH 11/11] jfs: " Luis R. Rodriguez
2017-11-30 16:36 ` [PATCH 00/11] fs: use freeze_fs on suspend/hibernate Yu Chen
2017-11-30 16:41   ` Jiri Kosina
2017-11-30 16:50     ` Yu Chen
2017-12-01 19:05     ` Jeff Layton
2017-12-01 21:51       ` Dave Chinner
2017-11-30 17:01 ` Bart Van Assche
2017-11-30 19:42   ` Luis R. Rodriguez
2017-11-30 20:53     ` Bart Van Assche
2017-11-30 21:03       ` Dave Chinner
2017-11-30 21:51 ` Pavel Machek
2017-12-01  0:44   ` Luis R. Rodriguez
2017-12-13  1:09 ` Rafael J. Wysocki
2017-12-19 16:50   ` Luis R. Rodriguez

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