Re: [PATCH v2 07/23] mips: update_mmu_cache() can replace __update_tlb()

[Nathan Chancellor <nathan@kernel.org>]

On Wed, Jun 14, 2023 at 10:43:30PM -0700, Hugh Dickins wrote:
> On Wed, 14 Jun 2023, Hugh Dickins wrote:
> > On Wed, 14 Jun 2023, Nathan Chancellor wrote:
> > > 
> > > I just bisected a crash while powering down a MIPS machine in QEMU to
> > > this change as commit 8044511d3893 ("mips: update_mmu_cache() can
> > > replace __update_tlb()") in linux-next.
> > 
> > Thank you, Nathan, that's very helpful indeed.  This patch certainly knew
> > that it wanted testing, and I'm glad to hear that it is now seeing some.
> > 
> > While powering down?  The messages below look like it was just coming up,
> > but no doubt that's because you were bisecting (or because I'm unfamiliar
> > with what messages to expect there).  It's probably irrelevant information,
> > but I wonder whether the (V)machine worked well enough for a while before
> > you first powered down and spotted the problem, or whether it's never got
> > much further than trying to run init (busybox)?  I'm trying to get a feel
> > for whether the problem occurs under common or uncommon conditions.

Ugh sorry, I have been looking into too many bugs lately and got my
wires crossed :) this is indeed a problem when running init (which is
busybox, this is a simple Buildroot file system).

> > > Unfortunately, I can still
> > > reproduce it with the existing fix you have for this change on the
> > > mailing list, which is present in next-20230614.
> > 
> > Right, that later fix was only for a build warning, nothing functional
> > (or at least I hoped that it wasn't making any functional difference).
> > 
> > Thanks a lot for the detailed instructions below: unfortunately, those
> > would draw me into a realm of testing I've never needed to enter before,
> > so a lot of time spent on setup and learning.  Usually, I just stare at
> > the source.
> > 
> > What this probably says is that I should revert most my cleanup there,
> > and keep as close to the existing code as possible.  But some change is
> > needed, and I may need to understand (or have a good guess at) what was
> > going wrong, to decide what kind of retreat will be successful.
> > 
> > Back to the source for a while: I hope I'll find examples in nearby MIPS
> > kernel source (and git history), which will hint at the right way forward.
> > Then send you a patch against next-20230614 to try, when I'm reasonably
> > confident that it's enough to satisfy my purpose, but likely not to waste
> > your time.
> 
> I'm going to take advantage of your good nature by attaching
> two alternative patches, either to go on top of next-20230614.
> 
> mips1.patch,
>  arch/mips/mm/tlb-r4k.c |   12 +-----------
>  1 file changed, 1 insertion(+), 11 deletions(-)
> 
> is by far my favourite.  I couldn't see anything wrong with what's
> already there for mips, but it seems possible that (though I didn't
> find it) somewhere calls update_mmu_cache_pmd() on a page table.  So
> mips1.patch restores the pmd_huge() check, and cleans up further by
> removing the silly pgdp, p4dp, pudp, pmdp stuff: the pointer has now
> been passed in by the caller, why walk the tree again?  I should have
> done it this way before.
> 
> But if that doesn't work, then I'm afraid it will have to be
> mips2.patch,
>  arch/mips/include/asm/pgtable.h |   15 ++++++++++++---
>  arch/mips/mm/tlb-r3k.c          |    5 ++---
>  arch/mips/mm/tlb-r4k.c          |   27 ++++++++++++++++++---------
>  3 files changed, 32 insertions(+), 15 deletions(-)
> 
> which reverts all of the original patch and its build warning fix,
> and does a pte_unmap() to balance the silly pte_offset_map() there;
> with an apologetic comment for this being about the only place in
> the tree where I have no idea what to do if ptep were NULL.
> 
> I do hope that you find the first fixes the breakage; but if not, then

I hate to be the bearer of bad news but the first patch did not fix the
breakage, I see the same issue.

> I even more fervently hope that the second will, despite my hating it.
> Touch wood for the first, fingers crossed for the second, thanks,

Thankfully, the second one does. Thanks for the quick and thoughtful
responses!

Cheers,
Nathan