On Fri, May 18, 2018 at 01:49:12PM -0400, Kent Overstreet wrote: > On Fri, May 18, 2018 at 01:45:36PM -0400, Josef Bacik wrote: > > 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, > > That sounds pretty cool... > > What about being able to add a random fault injection point in the middle of an > existing function? Being able to stick race_fault() in random places was a > pretty big win in terms of getting good code coverage out of realistic tests. There's nothing stopping us from doing that, it just uses a kprobe to override the function with our helper, so we could conceivably put it anywhere in the function. The reason I limited it to individual functions was because it was easier than trying to figure out the side-effects of stopping mid-function. If I needed to fail mid-function I just added a helper where I needed it and failed that instead. I imagine safety is going to be of larger concern if we allow bpf scripts to randomly return anywhere inside a function, even if the function is marked as allowing error injection. Thanks, Josef
