Re: [PATCH 2/2] ext4: simulate various I/O and checksum errors when reading metadata

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, Nov 21, 2019 at 08:00:26PM -0500, Theodore Y. Ts'o wrote:
> On Thu, Nov 21, 2019 at 04:09:33PM -0800, Darrick J. Wong wrote:
> > > +static inline int ext4_simulate_fail(struct super_block *sb,
> > > +				     unsigned long flag)
> > 
> > Nit: bool?
> 
> Sure, I'll do this for the next version.
> 
> > If I'm reading this correctly, this means that userspace sets a
> > s_simulate_fail bit via sysfs knob, and the next time the filesystem
> > calls ext4_simulate_fail with the same bit set in @flag we'll return
> > true to say "simulate the failure" and clear the bit in s_simulate_fail?
> > 
> > IOWs, the simulated failures have to be re-armed every time?
> 
> Yes, that's correct.
> 
> > Seems reasonable, but consider the possibility that in the future it
> > might be useful if you could set up periodic failures (e.g. directory
> > lookups fail 10% of the time) so that you can see how something like
> > fsstress reacts to less-predictable failures?
> 
> So in theory, we could do that with dm_flakey --- but that's a pain in
> the tuckus, since you have to specify the LBA for the directory blocks
> that you might want to have fail.

Funny, I've been working on a fstests helper function to make it easy to
set up dm-flakey based on fiemap/getfsmap output and such. :)

> I implemented this so I could have
> a quick and dirty way of testing the first patch in this series (and
> in fact, I found a bug in the first version of the previous patch, so
> I'm glad I spent the time to implement the test patch :-).

Heh, cool!

> What might be interesting to do is some kind of eBPF hook where we
> pass in the block #, inode #, and metadata type, and the ePBF program
> could do use a much more complex set of criteria in terms of whether
> or not to trigger an EIO, or how to fuzz a particular block to either
> force a CRC failure, or to try to find bugs ala Hydra[1] (funded via a
> Google Faculty Research Award grant), but using a much more glass-box
> style test approach.

That would be fun.  Attach an arbitrary eBPF program to a range of
sectors.  I wonder how loud the howls of protest would be for "can we
let ebpf programs scribble on a kernel io buffer pleeze?"...

...a couple of years ago I sent out an RFCRAP patch so that you could
use eBPF's "new" ability to change function return values, which
Christoph immediately NAKd.  I think Josef's original purpose was so
that he could inject arbitrary debugging knobs all over btrfs.

> [1] https://gts3.org/~sanidhya/pubs/2019/hydra.pdf
> 
> This would be a lot more work, and I'm not sufficiently up to speed
> with eBPF, and I just needed a quick and dirty testing scheme.
> 
> The reason why I think it's worthwhile to land this patch (as opposed
> to throwing it away after doing the development work for the previous
> patch) is that it's a relatively small set of changes, and all of the
> code disappears if CONFIG_DEBUG_EXT4 is not enabled.  So it has no
> performance cost on production kernels, and it's highly unlikely that
> users would have a reason to use this feature on production use cases,
> so ripping this out if and when we have a more functional eBPF testing
> infrastructure to replace it shouldn't really be a problem.

Admittedly it's a debug knob so I don't see it as a big deal if you
merge this and some day rip it out or supersede it.  The XFS knobs have
undergone a few, uh, interface revisions.

> 					- Ted
> 
> P.S.  A fascinating question is whether we could make the hooks for
> this hypothetical eBPF hook general enough that it could work for more
> than just ext4, but for other file systems.  The problem is that the
> fs metadata types are not going to be same across different file
> systems, so that makes the API design quite tricky; and perhaps not
> worth it?

Yeah.  I mean, it's eBPF glomming onto random parts of the kernel, so I
don't think there's ever going to be a General API For Brain Slugs[3].

OTOH I need LSF topics so sure lets roll.

--D

[2] https://lore.kernel.org/linux-xfs/20171213061825.GO19219@magnolia/
[3] https://futurama.fandom.com/wiki/Brain_Slug