On Sat, Aug 27, 2016 at 10:43:18AM +1000, Dave Chinner wrote: > On Fri, Aug 26, 2016 at 07:04:56PM +0800, Zorro Lang wrote: > > When I try to do below steps(a V5 xfs on $fsile): > > > > # xfs_db -x -c "sb 0" -c "write features_ro_compat 4096" $fsfile > > # xfs_db -x -c "sb 0" -c "write features_ro_compat 0" $fsfile > > # xfs_db -c "sb 0" -c p $fsfile > > > > The step#2 and #3 all failed, as: > > > > Superblock has unknown read-only compatible features (0x1000) enable > > As it should. xfs_db *in expert mode* allows you to shoot yourself > in the foot. However, it doesn't guarantee you'll have the expertise > to be able to fix the hole you just shot in your foot. > > You have much to learn, grasshopper. :P So I wrote a *stupid* patch at here... HaHa, I'm not afraid to try more *this* if I can learn more:) Thanks so much you take time to explain lots of things for me. That's my pleasure:) And they're very helpful for me, I'm trying to read more man-pages and code for understand all you said below(Looks like I have much things to do this weekend:-P) Thanks, Zorro > > > If we break the superblock, at least xfs_db should help to print the > > superblock info. > > It should. You tried to write to it in expert mode, though. Try just > printing the field in read-only mode (-r).... > > > And as a xfs debugger, xfs_db should can ignore > > unexpected errors to continue the "expert" command which an expert > > want to do. > > You're making the assumption that "-x" makes the user an expert. > That is far from the truth - it just means someone read a man page, > not that they have any specific XFS knowledge. The talk I > gave at LCA 2015 is relevant here: > > https://www.youtube.com/watch?v=VpuVDfSXs-g > > I go into a bit of depth about how cognitive biases affect how > people learn and assess their levels of expertise. This should > explain why "expert" mode still needs /some/ safeguards. > > Remember: feature flags are the most critical part of the on-disk > format because they tell everything that parses the on-disk format > what format exists on disk. If it is changed to something the tools > don't recognise, the tools should /absolutely refuse/ to run in > anything other than the mode indicated by the feature flags (i.e. > read-only or not at all). > > This, however, only turns off part of xfs_db's "make it > easy-for-non-experts" automatic type detection functionality. If you > know what you are doing and how xfs_db works (i.e. the user is an > expert), this is trivial to get around. > > So, let me demonstrate: > > $ sudo xfs_db -x -c "sb 0" -c "write features_ro_compat 4096" /dev/vdc > features_ro_compat = 0x1000 > $ sudo xfs_db -x -c "sb 0" -c "write features_ro_compat 0" /dev/vdc > Superblock has unknown read-only compatible features (0x1000) enabled. > Attempted to mount read-only compatible filesystem read-write. > Filesystem can only be safely mounted read only. > no current type > $ > > Ok, there's an error there that tells me I can't decode the buffer > that was loaded because automatic type detection was not run. So, > lets load it as a raw buffer and set the type manually: > > $ sudo xfs_db -r -c "daddr 0" -c "type sb" -c "p features_ro_compat" /dev/vdc > Superblock has unknown read-only compatible features (0x1000) enabled. > Attempted to mount read-only compatible filesystem read-write. > Filesystem can only be safely mounted read only. > features_ro_compat = 0x1000 > > Yup, there we go, access to the superblock type parsing is > re-enabled by starting with a raw data buffer, then setting the type > manually. We probably should fix userspace to then remount in > read-only mode so this works correctly without needing this step. > (It's probably just a libxfs init flag that is wrong.) > > So: > > $ sudo xfs_db -x -c "daddr 0" -c "type sb" -c "write features_ro_compat 0" /dev/vdc > Superblock has unknown read-only compatible features (0x1000) enabled. > Attempted to mount read-only compatible filesystem read-write. > Filesystem can only be safely mounted read only. > features_ro_compat = 0 > $ sudo xfs_db -r -c "sb 0" -c "p features_ro_compat" /dev/vdc > features_ro_compat = 0 > $ > > Yup, I just reset it to zero with expert mode, simply by knowing how > xfs_db works and avoiding the error that occurred with automatic type > detection. But even if I can't set the type manually, I can still > fix this by going back to first principles. > > $ sudo xfs_db -x -c "sb 0" -c "write features_ro_compat 4096" /dev/vdc > features_ro_compat = 0x1000 > $ > > First - manually find the on-disk format structure offset: > > $ pahole fs/xfs/libxfs/xfs_sb.o |grep sb_features_ro_compat > __uint32_t sb_features_ro_compat; /* 212 4 */ > __be32 sb_features_ro_compat; /* 212 4 */ > $ > > Now we can just write zeros directly to the raw buffer, then check > it by re-reading the superblock using the automatic type detection: > > $ sudo xfs_db -x -c "daddr 0" -c "write fill 0 212 4" -c "sb 0" -c "p features_ro_compat" /dev/vdc > features_ro_compat = 0 > > Yup, problem solved. But: > > $ sudo xfs_db -r -c "sb 0" -c "p features_ro_compat" /dev/vdc > Metadata CRC error detected at xfs_sb block 0x0/0x200 > features_ro_compat = 0 > > It's not a clean solution for v5 filesystems - I have to recalc the > CRC. Previous I would have simply run xfs_repair to fix this, but > now I can do this: > > $ sudo xfs_db -x -c "sb 0" -c "crc" -c "crc -r" /dev/vdc > Verifying CRC: > crc = 0xdea3392d (bad) > Recalculating CRC: > crc = 0x3a7763c8 (correct) > $ sudo xfs_db -r -c "sb 0" -c "p features_ro_compat" /dev/vdc > features_ro_compat = 0 > $ > > And now it's all good. > > So, as you can see we have /multiple ways/ we can fix bad feature > fields with xfs_db. None of them require magic force flags, but it > does require the ability to solve problems from first principles. > > Cheers, > > Dave. > -- > Dave Chinner > david@xxxxxxxxxxxxx > > _______________________________________________ > xfs mailing list > xfs@xxxxxxxxxxx > http://oss.sgi.com/mailman/listinfo/xfs _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
