On Fri, May 18, 2018 at 03:48:58AM -0400, Kent Overstreet wrote: > These are all the remaining patches in my bcachefs tree that touch stuff > outside > fs/bcachefs. Not all of them are suitable for inclusion as is, I wanted to get > some discussion first. > > * pagecache add lock > > This is the only one that touches existing code in nontrivial ways. The > problem > it's solving is that there is no existing general mechanism for shooting down > pages in the page and keeping them removed, which is a real problem if you're > doing anything that modifies file data and isn't buffered writes. > > Historically, the only problematic case has been direct IO, and people have > been > willing to say "well, if you mix buffered and direct IO you get what you > deserve", and that's probably not unreasonable. But now we have fallocate > insert > range and collapse range, and those are broken in ways I frankly don't want to > think about if they can't ensure consistency with the page cache. > > Also, the mechanism truncate uses (i_size and sacrificing a goat) has > historically been rather fragile, IMO it might be a good think if we switched > it > to a more general rigorous mechanism. > > I need this solved for bcachefs because without this mechanism, the page cache > inconsistencies lead to various assertions popping (primarily when we didn't > think we need to get a disk reservation going by page cache state, but then do > the actual write and disk space accounting says oops, we did need one). And > having to reason about what can happen without a locking mechanism for this is > not something I care to spend brain cycles on. > > That said, my patch is kind of ugly, and it requires filesystem changes for > other filesystems to take advantage of it. And unfortunately, since one of the > code paths that needs locking is readahead, I don't see any realistic way of > implementing the locking within just bcachefs code. > > So I'm hoping someone has an idea for something cleaner (I think I recall > Matthew Wilcox saying he had an idea for how to use xarray to solve this), but > if not I'll polish up my pagecache add lock patch and see what I can do to > make > it less ugly, and hopefully other people find it palatable or at least useful. > > * lglocks > > They were removed by Peter Zijlstra when the last in kernel user was removed, > but I've found them useful. His commit message seems to imply he doesn't think > people should be using them, but I'm not sure why. They are a bit niche > though, > I can move them to fs/bcachefs if people would prefer. > > * Generic radix trees > > This is a very simple radix tree implementation that can store types of > arbitrary size, not just pointers/unsigned long. It could probably replace > flex arrays. > > * Dynamic fault injection >
I've not looked at this at all so this may not cover your usecase, but I implemeted a bpf_override_return() to do focused error injection a year ago. I have this script https://github.com/josefbacik/debug-scripts/blob/master/inject-error.py that does it generically, all you have to do is tag the function you want to be error injectable with ALLOW_ERROR_INJECTION() and then you get all these nice things like a debugfs interface to trigger them or use the above script to trigger specific errors and such. 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
