From: Josef Bacik <josef@toxicpanda.com>
To: Kent Overstreet <kent.overstreet@gmail.com>
Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
Andrew Morton <akpm@linux-foundation.org>,
Dave Chinner <dchinner@redhat.com>,
darrick.wong@oracle.com, tytso@mit.edu,
linux-btrfs@vger.kernel.org, clm@fb.com, jbacik@fb.com,
viro@zeniv.linux.org.uk, willy@infradead.org,
peterz@infradead.org
Subject: Re: [PATCH 00/10] RFC: assorted bcachefs patches
Date: Fri, 18 May 2018 13:45:36 -0400 [thread overview]
Message-ID: <20180518174536.ai26bg3bhlvzq4pi@destiny> (raw)
In-Reply-To: <20180518074918.13816-1-kent.overstreet@gmail.com>
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
next prev parent reply other threads:[~2018-05-18 17:45 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-05-18 7:48 [PATCH 00/10] RFC: assorted bcachefs patches Kent Overstreet
2018-05-18 7:49 ` [PATCH 01/10] mm: pagecache add lock Kent Overstreet
2018-05-18 13:13 ` Matthew Wilcox
2018-05-18 15:53 ` Christoph Hellwig
2018-05-18 17:45 ` Kent Overstreet
2018-05-23 23:55 ` Notes on locking for pagacache consistency (was: [PATCH 01/10] mm: pagecache add lock) Kent Overstreet
2018-05-20 22:45 ` [PATCH 01/10] mm: pagecache add lock Kent Overstreet
2018-05-23 15:22 ` Christoph Hellwig
2018-05-23 17:12 ` Kent Overstreet
2018-05-18 7:49 ` [PATCH 02/10] mm: export find_get_pages() Kent Overstreet
2018-05-18 16:00 ` Christoph Hellwig
2018-05-18 7:49 ` [PATCH 03/10] locking: bring back lglocks Kent Overstreet
2018-05-18 9:51 ` Peter Zijlstra
2018-05-18 10:13 ` Kent Overstreet
2018-05-18 11:03 ` Peter Zijlstra
2018-05-18 11:39 ` Kent Overstreet
2018-05-18 7:49 ` [PATCH 04/10] locking: export osq_lock()/osq_unlock() Kent Overstreet
2018-05-18 9:52 ` Peter Zijlstra
2018-05-18 10:18 ` Kent Overstreet
2018-05-18 11:08 ` Peter Zijlstra
2018-05-18 11:32 ` Kent Overstreet
2018-05-18 11:40 ` Peter Zijlstra
2018-05-18 12:40 ` Kent Overstreet
2018-05-18 7:49 ` [PATCH 05/10] don't use spin_lock_irqsave() unnecessarily Kent Overstreet
2018-05-18 16:01 ` Christoph Hellwig
2018-05-18 7:49 ` [PATCH 06/10] Generic radix trees Kent Overstreet
2018-05-18 16:02 ` Christoph Hellwig
2018-05-18 17:38 ` Kent Overstreet
2018-05-18 7:49 ` [PATCH 07/10] bcache: optimize continue_at_nobarrier() Kent Overstreet
2018-05-18 7:49 ` [PATCH 08/10] bcache: move closures to lib/ Kent Overstreet
2018-05-18 16:02 ` Christoph Hellwig
2018-05-18 7:49 ` [PATCH 09/10] closures: closure_wait_event() Kent Overstreet
2018-05-18 7:49 ` [PATCH 10/10] Dynamic fault injection Kent Overstreet
2018-05-18 16:02 ` Christoph Hellwig
2018-05-18 17:37 ` Kent Overstreet
2018-05-18 19:05 ` Andreas Dilger
2018-05-18 19:10 ` Kent Overstreet
2018-05-18 20:54 ` Andreas Dilger
2018-05-18 7:55 ` [PATCH 00/10] RFC: assorted bcachefs patches Kent Overstreet
2018-05-18 17:45 ` Josef Bacik [this message]
2018-05-18 17:49 ` Kent Overstreet
2018-05-18 18:03 ` Josef Bacik
2018-05-18 18:28 ` Kent Overstreet
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