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
