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Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization

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On Tue, Apr 04 2017, Dave Chinner wrote:

> On Mon, Apr 03, 2017 at 04:00:55PM +0200, Jan Kara wrote:
>> On Sun 02-04-17 09:05:26, Dave Chinner wrote:
>> > On Thu, Mar 30, 2017 at 12:12:31PM -0400, J. Bruce Fields wrote:
>> > > On Thu, Mar 30, 2017 at 07:11:48AM -0400, Jeff Layton wrote:
>> > > > On Thu, 2017-03-30 at 08:47 +0200, Jan Kara wrote:
>> > > > > Because if above is acceptable we could make reported i_version to be a sum
>> > > > > of "superblock crash counter" and "inode i_version". We increment
>> > > > > "superblock crash counter" whenever we detect unclean filesystem shutdown.
>> > > > > That way after a crash we are guaranteed each inode will report new
>> > > > > i_version (the sum would probably have to look like "superblock crash
>> > > > > counter" * 65536 + "inode i_version" so that we avoid reusing possible
>> > > > > i_version numbers we gave away but did not write to disk but still...).
>> > > > > Thoughts?
>> > > 
>> > > How hard is this for filesystems to support?  Do they need an on-disk
>> > > format change to keep track of the crash counter?
>> > 
>> > Yes. We'll need version counter in the superblock, and we'll need to
>> > know what the increment semantics are. 
>> > 
>> > The big question is how do we know there was a crash? The only thing
>> > a journalling filesystem knows at mount time is whether it is clean
>> > or requires recovery. Filesystems can require recovery for many
>> > reasons that don't involve a crash (e.g. root fs is never unmounted
>> > cleanly, so always requires recovery). Further, some filesystems may
>> > not even know there was a crash at mount time because their
>> > architecture always leaves a consistent filesystem on disk (e.g. COW
>> > filesystems)....
>> 
>> What filesystems can or cannot easily do obviously differs. Ext4 has a
>> recovery flag set in superblock on RW mount/remount and cleared on
>> umount/RO remount.
>
> Even this doesn't help. A recent bug that was reported to the XFS
> list - turns out that systemd can't remount-ro the root
> filesystem sucessfully on shutdown because there are open write fds
> on the root filesystem when it attempts the remount. So it just
> reboots without a remount-ro. This uncovered a bug in grub in

Filesystems could use register_reboot_notifier() to get a notification
that even systemd cannot stuff-up.  It could check for dirty data and, if
there is none (which there shouldn't be if a sync happened), it does a
single write to disk to update the superblock (or a single write to each
disk... or something).
md does this, because getting the root device to be marked read-only is
even harder than getting the root filesystem to be remounted read-only.

NeilBrown

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Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization Jan Kara <jack@suse.cz> - 2017-03-29 13:20 +0200
  Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and  optimization Jeff Layton <jlayton@redhat.com> - 2017-03-29 20:00 +0200
    Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization Dave Chinner <david@fromorbit.com> - 2017-03-30 01:50 +0200
      Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and  optimization Jeff Layton <jlayton@redhat.com> - 2017-03-30 13:30 +0200
      Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization "J. Bruce Fields" <bfields@fieldses.org> - 2017-04-04 20:40 +0200
    Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization Jan Kara <jack@suse.cz> - 2017-03-30 08:50 +0200
      Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and  optimization Jeff Layton <jlayton@redhat.com> - 2017-03-30 13:20 +0200
        Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization "J. Bruce Fields" <bfields@fieldses.org> - 2017-03-30 18:20 +0200
          Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and  optimization Jeff Layton <jlayton@redhat.com> - 2017-03-30 20:40 +0200
            Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization Boaz Harrosh <openosd@gmail.com> - 2017-03-30 23:20 +0200
            Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization "J. Bruce Fields" <bfields@fieldses.org> - 2017-04-04 20:40 +0200
              Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization NeilBrown <neil@brown.name> - 2017-04-05 03:50 +0200
                Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization Jan Kara <jack@suse.cz> - 2017-04-05 10:10 +0200
                Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization "J. Bruce Fields" <bfields@fieldses.org> - 2017-04-05 20:20 +0200
                Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization NeilBrown <neil@brown.name> - 2017-04-06 03:20 +0200
                Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization Jan Kara <jack@suse.cz> - 2017-04-06 09:30 +0200
                Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization "J. Bruce Fields" <bfields@fieldses.org> - 2017-04-05 19:50 +0200
          Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization Dave Chinner <david@fromorbit.com> - 2017-04-02 01:10 +0200
            Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization Jan Kara <jack@suse.cz> - 2017-04-03 16:10 +0200
              Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization Dave Chinner <david@fromorbit.com> - 2017-04-04 14:40 +0200
                Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization "J. Bruce Fields" <bfields@fieldses.org> - 2017-04-04 20:00 +0200
                Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization NeilBrown <neil@brown.name> - 2017-04-05 03:30 +0200

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