Re: [PATCH 00/10] RFC: assorted bcachefs patches

[Josef Bacik]

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