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

[Steve Capper]

On 23 February 2016 at 18:47, Will Deacon <will.deacon at arm.com> wrote:
> [adding Steve, since he worked on THP for 32-bit ARM]

Apologies for my late reply...

>
> 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?

pmd_mknotpresent on arm looks inconsistent with the other
architectures and can be changed.

Having had a look at the usage, I can't see it causing an immediate
problem (that needs to be addressed by an emergency patch).
We don't have a notion of splitting pmds (so there is no splitting
information to lose), and the only usage I could see of
pmd_mknotpresent was:

pmdp_invalidate(vma, haddr, pmd);
pmd_populate(mm, pmd, pgtable);

In mm/huge_memory.c, around line 3588.

So we invalidate the entry (which puts down a faulting entry from
pmd_mknotpresent and invalidates tlb), then immediately put down a
table entry with pmd_populate.

I have run a 32-bit ARM test kernel and exacerbated THP splits (that's
what took me time), and I didn't notice any problems with 4.5-rc5.

Cheers,
-- 
Steve

>
> Will
>
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