On Fri, Jun 06, 2025 at 12:43:20PM +0200, Christian Brauner wrote: > On Mon, Jun 02, 2025 at 05:03:27PM -0700, Darrick J. Wong wrote: > > On Sun, Jun 01, 2025 at 09:02:25AM +1000, Dave Chinner wrote: > > > On Fri, May 30, 2025 at 08:38:47AM -0700, Darrick J. Wong wrote: > > > > On Fri, May 30, 2025 at 07:17:00AM +0200, Christian Brauner wrote: > > > > > On Wed, May 28, 2025 at 09:25:50PM -0700, Darrick J. Wong wrote: > > > > > > On Thu, May 29, 2025 at 10:50:01AM +0800, Yafang Shao wrote: > > > > > > > Hello, > > > > > > > > > > > > > > Recently, we encountered data loss when using XFS on an HDD with bad > > > > > > > blocks. After investigation, we determined that the issue was related > > > > > > > to writeback errors. The details are as follows: > > > > > > > > > > > > > > 1. Process-A writes data to a file using buffered I/O and completes > > > > > > > without errors. > > > > > > > 2. However, during the writeback of the dirtied pagecache pages, an > > > > > > > I/O error occurs, causing the data to fail to reach the disk. > > > > > > > 3. Later, the pagecache pages may be reclaimed due to memory pressure, > > > > > > > since they are already clean pages. > > > > > > > 4. When Process-B reads the same file, it retrieves zeroed data from > > > > > > > the bad blocks, as the original data was never successfully written > > > > > > > (IOMAP_UNWRITTEN). > > > > > > > > > > > > > > We reviewed the related discussion [0] and confirmed that this is a > > > > > > > known writeback error issue. While using fsync() after buffered > > > > > > > write() could mitigate the problem, this approach is impractical for > > > > > > > our services. > > > > > > > > > > > > > > Instead, we propose introducing configurable options to notify users > > > > > > > of writeback errors immediately and prevent further operations on > > > > > > > affected files or disks. Possible solutions include: > > > > > > > > > > > > > > - Option A: Immediately shut down the filesystem upon writeback errors. > > > > > > > - Option B: Mark the affected file as inaccessible if a writeback error occurs. > > > > > > > > > > > > > > These options could be controlled via mount options or sysfs > > > > > > > configurations. Both solutions would be preferable to silently > > > > > > > returning corrupted data, as they ensure users are aware of disk > > > > > > > issues and can take corrective action. > > > > > > > > > > > > > > Any suggestions ? > > > > > > > > > > > > Option C: report all those write errors (direct and buffered) to a > > > > > > daemon and let it figure out what it wants to do: > > > > > > > > > > > > https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfs-linux.git/log/?h=health-monitoring_2025-05-21 > > > > > > https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfsprogs-dev.git/log/?h=health-monitoring-rust_2025-05-21 > > > > > > > > > > > > Yes this is a long term option since it involves adding upcalls from the > > > > > > > > > > I hope you don't mean actual usermodehelper upcalls here because we > > > > > should not add any new ones. If you just mean a way to call up from a > > > > > lower layer than that's obviously fine. > > > > > > > > Correct. The VFS upcalls to XFS on some event, then XFS queues the > > > > event data (or drops it) and waits for userspace to read the queued > > > > events. We're not directly invoking a helper program from deep in the > > > > guts, that's too wild even for me. ;) > > > > > > > > > Fwiw, have you considered building this on top of a fanotify extension > > > > > instead of inventing your own mechanism for this? > > > > > > > > I have, at various stages of this experiment. > > > > > > > > Originally, I was only going to export xfs-specific metadata events > > > > (e.g. this AG's inode btree index is bad) so that the userspace program > > > > (xfs_healer) could initiate a repair against the broken pieces. > > > > > > > > At the time I thought it would be fun to experiment with an anonfd file > > > > that emitted jsonp objects so that I could avoid the usual C struct ABI > > > > mess because json is easily parsed into key-value mapping objects in a > > > > lot of languages (that aren't C). It later turned out that formatting > > > > the json is rather more costly than I thought even with seq_bufs, so I > > > > added an alternate format that emits boring C structures. > > > > > > > > Having gone back to C structs, it would be possibly (and possibly quite > > > > nice) to migrate to fanotify so that I don't have to maintain a bunch of > > > > queuing code. But that can have its own drawbacks, as Ted and I > > > > discovered when we discussed his patches that pushed ext4 error events > > > > through fanotify: > > > > > > > > For filesystem metadata events, the fine details of representing that > > > > metadata in a generic interface gets really messy because each > > > > filesystem has a different design. > > > > > > Perhaps that is the wrong approach. The event just needs to tell > > > userspace that there is a metadata error, and the fs specific agent > > > that receives the event can then pull the failure information from > > > the filesystem through a fs specific ioctl interface. > > > > > > i.e. the fanotify event could simply be a unique error, and that > > > gets passed back into the ioctl to retreive the fs specific details > > > of the failure. We might not even need fanotify for this - I suspect > > > that we could use udev events to punch error ID notifications out to > > > userspace to trigger a fs specific helper to go find out what went > > > wrong. > > > > I'm not sure if you're addressing me or brauner, but I think it would be > > even simpler to retain the current design where events are queued to our > > special xfs anonfd and read out by userspace. Using fanotify as a "door > > bell" to go look at another fd is ... basically poll() but far more > > complicated than it ought to be. Pounding udev with events can result > > in userspace burning a lot of energy walking the entire rule chain. > > I don't think we need to rush any of this. My main concern is that if we > come up with something then I want it to be able to be used by other > filesystems as this seems something that is generally very useful. By > using fanotify we implicitly enable this which is why I'm asking. > > I don't want the outcome to be that there's a filesystem with a very > elaborate and detailed scheme that cannot be used by another one and > then we end up with slightly different implementations of the same > underlying concept. And so it will be impossible for userspace to > consume correctly even if abstracted in multiple libraries. Hrm. I 60% agree and 60% disagree with you. :D 60% disagree: for describing problems with internal filesystem metadata, I don't think there's a generic way to expose that outside of ugly stringly-parsing things like json. Frankly I don't think any fs project is going to want a piece of that cake. Maybe we can share the mechanism for returning fs-specific metadata error information to a daemon, but the structure of the data is going to be per-filesystem. And I think the only clients are going to be written by the same fs folks for internal purposes like starting online fsck. 60% agree: for telling most programs that "hey, something went wrong with this file range", I think it's completely appropriate to fling that out via the existing generic fsnotify mechanisms that ext4 wired up. I think the same applies to sending a "your fs is broken" event via fsnotify too, in case regular user programs decide they want to nope out. IIRC there's already a generic notification for that too. Fortunately the vfs hooks I wrote for xfs_healer are general enough that I don't think it'd be difficult to wire them up to fsnotify. > I think udev is the wrong medium for this and I'm pretty sure that the > udev maintainers agree with me on this. > > I think this specific type of API would really benefit from gathering > feedback from userspace. There's All Systems Go in Berlin in September > and that might not be the worst time to present what you did and give a > little demo. I'm not sure how fond you are of traveling though rn: > https://all-systems-go.io/ I like travelling! But happily, I'll be travelling for most of September already. But yeah, I've wondered if it would be useful to write a generic service that would hang around on dbus, listen for the fsnotify events, and broadcast them to clients. I suspect that sifting through all the containerization and idmapping stuff so that app A can't hear about errors in app B's container might be a lot of work though. --D > > > Keeping unprocessed failures in an internal fs queue isn't a big > > > deal; it's not a lot of memory, and it can be discarded on unmount. > > > At that point we know that userspace did not care about the > > > failure and is not going to be able to query about the failure in > > > future, so we can just throw it away. > > > > > > This also allows filesystems to develop such functionality in > > > parallel, allowing us to find commonality and potential areas for > > > abstraction as the functionality is developed, rahter than trying to > > > come up with some generic interface that needs to support all > > > possible things we can think of right now.... > > > > Agreed. I don't think Ted or Jan were enthusiastic about trying to make > > a generic fs metadata event descriptor either.
