On Tue, Nov 30, 2010 at 10:03:58AM +0800, liubo wrote: > On 11/30/2010 04:10 AM, Josef Bacik wrote: > > On Thu, Nov 25, 2010 at 05:52:47PM +0800, Miao Xie wrote: > >> Btrfs has a number of BUG_ON()s, which may lead btrfs to unpleasant panic. > >> Meanwhile, they are very ugly and should be handled more propriately. > >> > >> There are mainly two ways to deal with these BUG_ON()s. > >> > >> 1. For those errors which can be handled well by callers, we just return > >> their > >> error number to callers. > >> > >> 2. For others, We can force the filesystem readonly when it hits errors, > >> which > >> is what this patchset has done. Replaced BUG_ON() with the interface > >> provided > >> in this patchset, we will get error infomation via dmesg. Since btrfs is > >> now > >> readonly, we can save our data safely and umount it, then a btrfsck is > >> recommended. > >> > >> By these ways, we can protect our filesystem from panic caused by those > >> BUG_ONs. > >> > >> --- > >> fs/btrfs/ctree.h | 21 ++++++++++ > >> fs/btrfs/disk-io.c | 23 +++++++++++ > >> fs/btrfs/super.c | 100 > >> ++++++++++++++++++++++++++++++++++++++++++++++- > >> fs/btrfs/transaction.c | 7 +++ > >> 4 files changed, 148 insertions(+), 3 deletions(-) > >> > > > > Overall seems sane, but what about kernels that don't make these checks? > > I'm ok > > with "well sucks for them" as an answer, just want to make sure we've at > > least > > though about it. > > You mean those code that does nothing on ret-checks? > > IMO, if the code really needs ret-check, we should deal with them seriously, > or just > leave it alone. And this is a step-by-step job. >
Sorry I mean for older kernels that don't know about these "hey your fs is screwed" flags. It seems like they'll just get ignored, are we sure thats what we want to happen? I'm fine with that, but if we don't want that to happen it may be good to have a incompat flag. > > > > Also I'm not sure marking the fs as broken is the right move here. Ext3/4 > > don't > > do this, they just mount read-only, as long as you can still unmount the > > filesystem everything comes out ok. Think of the case where we just get a > > spurious EIO, the fs should be fine the next time around, there's reason to > > force the user to run fsck in this case. > > > > Yes, I agree on this. > For spurious EIO, it mainly depends on coders, returning the errno to caller > may work on > bypassing fsck. > Right I'm worried about the flipping read only stuff being kicked by EIO, which happens with ext* and could happen with btrfs in the right cases. I'm not saying thats wrong, its what should happen, I'm just saying we need to be able to unmount the filesystem and mount it back up without needing to run an fsck in between. > While I'm working on this readonly stuff, it is difficult to solve the > potential > deadlock when we write the super block to disk. > Since btrfs supports multi-device, before write-super, we must get the device > lock > "device_list_mutex" first, and this has puzzled me a lot. > > BTW, I've tried another way to bypass deadlock. I made the write-super stuff > into umount, > which can make us free from deadlock, however, while testing this, it seemes > that umount > cannot work due to a ext3/4 jbd oops, I'm digging on this oops... > > So, any ideas about free from deadlock? > None :). The best thing I can think of is do like we're doing with the read only stuff and only write out the super right before we flip read only, and then make umount make sure that if we're mounted read only to not do anything. Truth be told I hate this "mark the fs as broken" idea. We don't know if the error we got means the filesystem is broken (for example the EIO case). If we do hit actual corruption maybe it would be good, and in that case we should write out the super at the point we find that corruption and then flip read only and have that be the only time we have to worry about writing out the super. So I guess that's 2 options 1) Ditch the "the fs is broken" flag. This makes things nice and simple since on-disk is already consistent, all we have to do is drop anything thats dirty and we're home free. 2) Keep the flag, but only worry about writing it out on a case by case basis. So we have a btrfs_corrupt_fs() function that writes out the super with the appropriate flag, and then flips the fs read only. Then we don't have to do anything special in the common paths, just the normal "hey is this fs read only?" things, so for all other cases we can just flip the fs read only and everything works. I hope that makes sense, if not feel free to ignore me and just keep doing what you've been doing :). Thanks, Josef -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
