Re: [PATCH] ext4: add fallocate mode blocking for debugging purposes

[Theodore Ts'o <tytso@mit.edu>]

On Tue, Apr 15, 2014 at 11:15:41AM -0500, Eric Sandeen wrote:
> 
> I tend to agree, better to fix the kernel than to add a knob to turn it
> off.  And fsx changes can happen a lot quicker than kernel changes.  [1]
> 
> And if it's really unsafe, and you really want to add a knob, I'd at least
> default it to off until it's non-corrupting, and add a message that
> this tunable will go away as soon as it's stable, so you'll have no
> qualms about quickly deprecating it...

Yeah, I went back and forth on this.  One of there reasons why I added
a kernel knob is that *I* can make the kernel change a lot faster than
it would be to tweak all of the various xfstests program to globally
disable certain operations in fsx, fstress, etc.

I also had a sneaking suspicion that we might have a similar issue
with the INSERT RANGE patches which are coming down the pike, and so
having a general way of also being able INSERT RANGE if to be able to
quickly determine whether a potential bug was caused by INSERT RANGE
or some other pending changes might also be useful.

I freely admit it is a bit of a hack, though.  Does the hack smell
less bad if we wrap it in CONFIG_EXT4FS_DEBUG?

> [1] it'd be nifty to make an env. var in xfstests which can globally
> disable certain fsx operations across all tests which run fsx...

Yes, although as I mentioned above, it would be really nice if it
worked across all of the various tests, and not just be limited to
fsx, or even just fsx and fstress.

					- Ted