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

From: Dave Chinner
Date: Mon Apr 16 2012 - 20:43:34 EST

On Mon, Apr 16, 2012 at 03:02:50PM -0700, 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...
> 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)

Unlinked-but-open files are the reason that XFS dirties the log
after the freeze process is complete. This ensures that if the
system crashes while the filesystem is frozen then log recovery
during the next mount will process the unlinked (orphaned) inodes
and free the correctly. i.e. you can still freeze a filesystem with
inodes in this state successfully and have everythign behave as
you'd expect.

I'm not sure how other filesystems handle this problem, but perhaps
pushing this check down into filesystem specific code or adding a
superblock feature flag might be a way to allow filesystems to
handle this case in the way they think is best...


Dave Chinner
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