Re: [PATCH 00/19 v5] Fix filesystem freezing deadlocks

From: Joel Becker
Date: Tue Apr 17 2012 - 15:35:20 EST

On Tue, Apr 17, 2012 at 11:32:46AM +0200, Jan Kara wrote:
> On Mon 16-04-12 15:02:50, Andreas Dilger wrote:
> > On 2012-04-16, at 9:13 AM, Jan Kara wrote:
> > > Another potential contention point might be patch 19. In that patch
> > > we make freeze_super() refuse to freeze the filesystem when there
> > > are open but unlinked files which may be impractical in some cases.
> > > The main reason for this is the problem with handling of file deletion
> > > from fput() called with mmap_sem held (e.g. from munmap(2)), and
> > > then there's the fact that we cannot really force such filesystem
> > > into a consistent state... But if people think that freezing with
> > > open but unlinked files should happen, then I have some possible
> > > solutions in mind (maybe as a separate patchset since this is
> > > large enough).
> >
> > Looking at a desktop system, I think it is very typical that there
> > are open-unlinked files present, so I don't know if this is really
> > an acceptable solution. It isn't clear from your comments whether
> > this is a blanket refusal for all open-unlinked files, or only in
> > some particular cases...
> Thanks for looking at this. It is currently a blanket refusal. And I
> agree it's problematic. There are two problems with open but unlinked
> files.

Let me add my name to the chorus of "we have to handle freezing
with open+unlinked, we cannot assume they don't exist."

> One is that some old filesystems cannot get in a consistent state in
> presence of open but unlinked files but for filesystems we really care
> about - xfs, ext4, ext3, btrfs, or even ocfs2, gfs2 - that is not a real
> issue (these filesystems will delete those inodes on next mount read-write).

Others have pointed out that we can flag the safe filesystems.
I'd even be willing to say you can't freeze the unsafe filesystems.

> The other problem is with what should happen when you put last inode
> reference on a frozen filesystem. Two possibilities I see are:
> a) block the iput() call - that is inconvenient because it can be
> called in various contexts. I think we could possibly use the same level of
> freeze protection as for page fault (this has changed since I originally
> thought about this and that would make things simpler) but I'm not
> completely sure.

Given that frozen filesystems can stay that way for a while,
couldn't that lead to a million frozen df(1)s? It's like your average
NFS network failure.

> b) let the iput finish but filesystem will keep inode on its orphan list
> (or it's equivalent) and the inode will be deleted after the filesystem is
> thawed. The advantage of this is we don't have to block iput(), the
> disadvantage is we have to have filesystem support and not all filesystems
> can do this.

Perhaps we handle iput() like unlinked. If the filesystem can
handle it, we allow it, otherwise we block.


> Any thoughts?
> Honza
> >
> > lsof | grep deleted
> > nautilus 25393 adilger 19r REG 253,0 340 253954 /home/adilger/.local/share/gvfs-metadata/home (deleted)
> > nautilus 25393 adilger 20r REG 253,0 32768 253964 /home/adilger/.local/share/gvfs-metadata/home-f332a8f3.log (deleted)
> > gnome-ter 25623 adilger 22u REG 0,18 17841 2717846 /tmp/vtePIRJCW (deleted)
> > gnome-ter 25623 adilger 23u REG 0,18 5568 2717847 /tmp/vteDCSJCW (deleted)
> > gnome-ter 25623 adilger 29u REG 0,18 480 2728484 /tmp/vte6C1TCW (deleted)
> --
> Jan Kara <jack@xxxxxxx>
> SUSE Labs, CR


"The first requisite of a good citizen in this republic of ours
is that he shall be able and willing to pull his weight."
- Theodore Roosevelt
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