[BUG] random kernel crashes after THP rework on s390 (maybe also on PowerPC and ARM)

[Will Deacon]

[adding Steve, since he worked on THP for 32-bit ARM]

On Tue, Feb 23, 2016 at 07:19:07PM +0100, Gerald Schaefer wrote:
> On Tue, 23 Feb 2016 13:32:21 +0300
> "Kirill A. Shutemov" <kirill at shutemov.name> wrote:
> > The theory is that the splitting bit effetely masked bogus pmd_present():
> > we had pmd_trans_splitting() in all code path and that prevented mm from
> > touching the pmd. Once pmd_trans_splitting() has gone, mm proceed with the
> > pmd where it shouldn't and here's a boom.
> 
> Well, I don't think pmd_present() == true is bogus for a trans_huge pmd under
> splitting, after all there is a page behind the the pmd. Also, if it was
> bogus, and it would need to be false, why should it be marked !pmd_present()
> only at the pmdp_invalidate() step before the pmd_populate()? It clearly
> is pmd_present() before that, on all architectures, and if there was any
> problem/race with that, setting it to !pmd_present() at this stage would
> only (marginally) reduce the race window.
> 
> BTW, PowerPC and Sparc seem to do the same thing in pmdp_invalidate(),
> i.e. they do not set pmd_present() == false, only mark it so that it would
> not generate a new TLB entry, just like on s390. After all, the function
> is called pmdp_invalidate(), and I think the comment in mm/huge_memory.c
> before that call is just a little ambiguous in its wording. When it says
> "mark the pmd notpresent" it probably means "mark it so that it will not
> generate a new TLB entry", which is also what the comment is really about:
> prevent huge and small entries in the TLB for the same page at the same
> time.
> 
> FWIW, and since the ARM arch-list is already on cc, I think there is
> an issue with pmdp_invalidate() on ARM, since it also seems to clear
> the trans_huge (and formerly trans_splitting) bit, which actually makes
> the pmd !pmd_present(), but it violates the other requirement from the
> comment:
> "the pmd_trans_huge and pmd_trans_splitting must remain set at all times
> on the pmd until the split is complete for this pmd"

I've only been testing this for arm64 (where I'm yet to see a problem),
but we use the generic pmdp_invalidate implementation from
mm/pgtable-generic.c there. On arm64, pmd_trans_huge will return true
after pmd_mknotpresent. On arm, it does look to be buggy, since it nukes
the entire entry... Steve?

Will