Re: [PATCH AUTOSEL 4.14 25/35] iomap: sub-block dio needs to zeroout beyond EOF

[Dave Chinner <david@fromorbit.com>]

On Thu, Nov 29, 2018 at 01:47:56PM +0100, Greg KH wrote:
> On Thu, Nov 29, 2018 at 11:14:59PM +1100, Dave Chinner wrote:
> > 
> > Cherry picking only one of the 50-odd patches we've committed into
> > late 4.19 and 4.20 kernels to fix the problems we've found really
> > seems like asking for trouble. If you're going to back port random
> > data corruption fixes, then you need to spend a *lot* of time
> > validating that it doesn't make things worse than they already
> > are...
> 
> Any reason why we can't take the 50-odd patches in their entirety?  It
> sounds like 4.19 isn't fully fixed, but 4.20-rc1 is?  If so, what do you
> recommend we do to make 4.19 working properly?

You coul dpull all the fixes, but then you have a QA problem.
Basically, we have multiple badly broken syscalls (FICLONERANGE,
FIDEDUPERANGE and copy_file_range), and even 4.20-rc4 isn't fully
fixed.

There were ~5 critical dedupe/clone data corruption fixes for XFS
went into 4.19-rc8.

There were ~30 patches that went into 4.20-rc1 that fixed the
FICLONERANGE/FIDEDUPERANGE ioctls. That completely reworks the
entire VFS infrastructure for those calls, and touches several
filesystems as well. It fixes problems with setuid files, swap
files, modifying immutable files, failure to enforce rlimit and
max file size constraints, behaviour that didn't match man page
descriptions, etc.

There were another ~10 patches that went into 4.20-rc4 that fixed
yet more data corruption and API problems that we found when we
enhanced fsx to use the above syscalls.

And I have another ~10 patches that I'm working on right now to fix
the copy_file_range() implementation - it has all the same problems
I listed above for FICLONERANGE/FIDEDUPERANGE and some other unique
ones. I'm currently writing error condition tests for fstests so
that we at least have some coverage of the conditions
copy_file_range() is supposed to catch and fail. This might all make
a late 4.20-rcX, but it's looking more like 4.21 at this point.

As to testing this stuff, I've spend several weeks now on this and
so has Darrick. Between us we've done a huge amount of QA needed to
verify that the problems are fixed and it is still ongoing. From
#xfs a couple of days ago:

[28/11/18 16:59] * djwong hits 6 billion fsxops...
[28/11/18 17:07] <dchinner_> djwong: I've got about 3.75 billion ops running on a machine here....
[28/11/18 17:20] <djwong> note that's 1 billion fsxops x 6 machines
[28/11/18 17:21] <djwong> [xfsv4, xfsv5, xfsv5 w/ 1k blocks] * [directio fsx, buffered fsx]
[28/11/18 17:21] <dchinner_> Oh, I've got 3.75B x 4 instances on one filesystem :P
[28/11/18 17:22] <dchinner_> [direct io, buffered] x [small op lengths, large op lengths]

And this morning:

[30/11/18 08:53] <djwong> 7 billion fsxops...

I stopped my tests at 5 billion ops yesterday (i.e. 20 billion ops
aggregate) to focus on testing the copy_file_range() changes, but
Darrick's tests are still ongoing and have passed 40 billion ops in
aggregate over the past few days.

The reason we are running these so long is that we've seen fsx data
corruption failures after 12+ hours of runtime and hundreds of
millions of ops. Hence the testing for backported fixes will need to
replicate these test runs across multiple configurations for
multiple days before we have any confidence that we've actually
fixed the data corruptions and not introduced any new ones.

If you pull only a small subset of the fixes, the fsx will still
fail and we have no real way of actually verifying that there have
been no regression introduced by the backport.  IOWs, there's a
/massive/ amount of QA needed for ensuring that these backports work
correctly.

Right now the XFS developers don't have the time or resources
available to validate stable backports are correct and regression
fre because we are focussed on ensuring the upstream fixes we've
already made (and are still writing) are solid and reliable.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com