Re: Regression in kernel 4.12-rc1 for Powerpc 32 - bisected to commit 3448890c32c3

From: Al Viro
Date: Thu Jun 22 2017 - 15:25:28 EST

On Thu, Jun 22, 2017 at 09:19:58AM -0500, Larry Finger wrote:

> > Ugh... MintPPC appears to be dead. On KVM with Debian userland (either
> > jessie or wheezy - no difference in result) booting the commit in
> > question with your .config oopses as soon as pata_macio is initialized,
> > due to the bug in "treewide: Move dma_ops from struct dev_archdata into
> > struct device", and after cherry-picking your own fix for that (commit
> > 46f401c4297a "powerpc/pmac: Fix crash in dma-mapping.h with NULL dma_ops")
> > the result boots just fine.
> >
> > Again, that happens both for Debian 8 and Debian 7 userlands, so unless
> > Mint had been doing something very odd there, I would question the accuracy
> > of your bisect...

> Any chance that real hardware differs from KVM emulation?

For that one? Bloody unlikely; udev could, theoretically, hit different codepaths
due to different devices being observed, etc., but changes in that commit are
not in the areas that would be easy to get wrong in emulator.

> All I know at this
> point is that commit f2ed8beb with 46f401c4 backported boots OK and commit
> 3448890c with the same backport fails.
> I will try loading jessie and see what happens.

I would recheck which kernels are being booted - I had screwed that up during long
bisects often enough...

BTW, could you try to check what happens if you kill the
if (__builtin_constant_p(n) && (n <= 8))
bits in raw_copy_{to,from}_user()? The usefulness of those (in __copy_from_user()
originally) had always been dubious and the things are simpler without them.
If _that_ turns out to cure breakage, I would be very surprised, though.