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
