Subject: Re: [PATCH] xfs: introduce "metasync" api to sync metadata to fsblock

From: Pingfan Liu <[email-masked]>
Date: Tue, 15 Oct 2019 10:12:42 +0800

On Mon, Oct 14, 2019 at 08:23:39AM -0500, Eric Sandeen wrote:
> On 10/14/19 4:43 AM, Jan Kara wrote:
> > On Mon 14-10-19 16:33:15, Pingfan Liu wrote:
> > > On Sun, Oct 13, 2019 at 09:34:17AM -0700, Darrick J. Wong wrote:
> > > > On Sun, Oct 13, 2019 at 10:37:00PM +0800, Pingfan Liu wrote:
> > > > > When using fadump (fireware assist dump) mode on powerpc, a mismatch
> > > > > between grub xfs driver and kernel xfs driver has been obsevered.  Note:
> > > > > fadump boots up in the following sequence: fireware -> grub reads kernel
> > > > > and initramfs -> kernel boots.
> > > > > 
> > > > > The process to reproduce this mismatch:
> > > > >    - On powerpc, boot kernel with fadump=on and edit /etc/kdump.conf.
> > > > >    - Replacing "path /var/crash" with "path /var/crashnew", then, "kdumpctl
> > > > >      restart" to rebuild the initramfs. Detail about the rebuilding looks
> > > > >      like: mkdumprd /boot/initramfs-`uname -r`.img.tmp;
> > > > >            mv /boot/initramfs-`uname -r`.img.tmp /boot/initramfs-`uname -r`.img
> > > > >            sync
> > > > >    - "echo c >/proc/sysrq-trigger".
> > > > > 
> > > > > The result:
> > > > > The dump image will not be saved under /var/crashnew/* as expected, but
> > > > > still saved under /var/crash.
> > > > > 
> > > > > The root cause:
> > > > > As Eric pointed out that on xfs, 'sync' ensures the consistency by writing
> > > > > back metadata to xlog, but not necessary to fsblock. This raises issue if
> > > > > grub can not replay the xlog before accessing the xfs files. Since the
> > > > > above dir entry of initramfs should be saved as inline data with xfs_inode,
> > > > > so xfs_fs_sync_fs() does not guarantee it written to fsblock.
> > > > > 
> > > > > umount can be used to write metadata fsblock, but the filesystem can not be
> > > > > umounted if still in use.
> > > > > 
> > > > > There are two ways to fix this mismatch, either grub or xfs. It may be
> > > > > easier to do this in xfs side by introducing an interface to flush metadata
> > > > > to fsblock explicitly.
> > > > > 
> > > > > With this patch, metadata can be written to fsblock by:
> > > > >    # update AIL
> > > > >    sync
> > > > >    # new introduced interface to flush metadata to fsblock
> > > > >    mount -o remount,metasync mountpoint
> > > > 
> > > > I think this ought to be an ioctl or some sort of generic call since the
> > > > jbd2 filesystems (ext3, ext4, ocfs2) suffer from the same "$BOOTLOADER
> > > > is too dumb to recover logs but still wants to write to the fs"
> > > > checkpointing problem.
> > > Yes, a syscall sounds more reasonable.
> > > > 
> > > > (Or maybe we should just put all that stuff in a vfat filesystem, I
> > > > don't know...)
> > > I think it is unavoidable to involve in each fs' implementation. What
> > > about introducing an interface sync_to_fsblock(struct super_block *sb) in
> > > the struct super_operations, then let each fs manage its own case?
> > 
> > Well, we already have a way to achieve what you need: fsfreeze.
> > Traditionally, that is guaranteed to put fs into a "clean" state very much
> > equivalent to the fs being unmounted and that seems to be what the
> > bootloader wants so that it can access the filesystem without worrying
> > about some recovery details. So do you see any problem with replacing
> > 'sync' in your example above with 'fsfreeze /boot && fsfreeze -u /boot'?
> > 
> > 								Honza
> 
> The problem with fsfreeze is that if the device you want to quiesce is, say,
> the root fs, freeze isn't really a good option.
Yes, that is the difference between my patch and fsfreeze.  But
honestly, it is a rare case where a system has not a /boot partition. Due
to the activity on /boot is very low, fsfreeze may meet the need, or
repeatly retry fsfress until success.
> 
> But the other thing I want to highlight about this approach is that it does not
> solve the root problem: something is trying to read the block device without
> first replaying the log.
> 
> A call such as the proposal here is only going to leave consistent metadata at
> the time the call returns; at any time after that, all guarantees are off again,
My patch places assumption that grub only accesses limited files and ensures the
consistency only on those files (kernel,initramfs).
> so the problem hasn't been solved.
Agree. The perfect solution should be a log aware bootloader.

Thanks and regards,
	Pingfan