Date:   Wed, 14 Jun 2023 22:43:30 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Nathan Chancellor <nathan@...nel.org>
cc:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>,
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        Peter Zijlstra <peterz@...radead.org>,
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        Will Deacon <will@...nel.org>,
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Subject: Re: [PATCH v2 07/23] mips: update_mmu_cache() can replace
 __update_tlb()

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

Hugh
View attachment "mips1.patch" of type "text/x-patch" (900 bytes)

View attachment "mips2.patch" of type "text/x-patch" (3927 bytes)

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