On Wed, Jan 31, 2024 at 10:37:18AM +1100, Dave Chinner wrote: > It should be obvious what has happened now - > current->journal_info is not null, so ext4 thinks it owns the > structure attached there and panics when it finds that it isn't an > ext4 journal handle being held there. > > I don't think there are any clear rules as to how filesystems can > and can't use current->journal_info. In general, a task can't jump > from one filesystem to another inside a transaction context like > this, so there's never been a serious concern about nested > current->journal_info assignments like this in the past. > > XFS is doing nothing wrong - we're allowed to define transaction > contexts however we want and use current->journal_info in this way. > However, we have to acknowledge that ext4 has also done nothing > wrong by assuming current->journal_info should below to it if it is > not null. Indeed, XFS does the same thing. Nice analysis. Fundamentally the current usage of current->journal_info assumes that a process would only be calling into one file system at a time. But obviously that's not going to be true in the case of one file system writing to memory which then triggers a page fault. As far as other potential avenues that could cause this kind of nesting, the other one which comes to mind might be sendfile(2) -- although in general the reader side won't trigger a transaction since the atime update tends to be done lazily. > The question here is what to do about this? The obvious solution is > to have save/restore semantics in the filesystem code that > sets/clears current->journal_info, and then filesystems can also do > the necessary "recursion into same filesystem" checks they need to > ensure that they aren't nesting transactions in a way that can > deadlock. > > Maybe there are other options - should filesystems even be allowed to > trigger page faults when they have set current->journal_info? Hmm, could XFS pre-fault target memory buffer for the bulkstat output before starting its transaction? Alternatively, ext4 could do a save of current->journal_info before starting to process the page fault, and restore it when it is done. Both of these seem a bit hacky, and the question is indeed, are there other avenues that might cause the transaction context nesting, such that a more general solution is called for? - Ted
