Re: frequent lockups in 3.18rc4

[Andy Lutomirski]

On Wed, Nov 19, 2014 at 2:56 PM, Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote:
> On Wed, Nov 19, 2014 at 10:56:26PM +0100, Thomas Gleixner wrote:
>> On Wed, 19 Nov 2014, Frederic Weisbecker wrote:
>> > I got a report lately involving context tracking. Not sure if it's
>> > the same here but the issue was that context tracking uses per cpu data
>> > and per cpu allocation use vmalloc and vmalloc'ed area can fault due to
>> > lazy paging.
>>
>> This is complete nonsense. pcpu allocations are populated right
>> away. Otherwise no single line of kernel code which uses dynamically
>> allocated per cpu storage would be safe.
>
> Note this isn't faulting because part of the allocation is swapped. No
> it's all reserved in the physical memory, but it's a lazy allocation.
> Part of it isn't yet addressed in the P[UGM?]D. That's what vmalloc_fault() is for.
>
> So it's a non-blocking/sleeping fault which is why it's probably fine
> most of the time except on code that isn't fault-safe. And I suspect that
> most people assume that kernel data won't fault so probably some other
> places have similar issues.
>
> That's a long standing issue. We even had to convert the perf callchain
> allocation to ad-hoc kmalloc() based per cpu allocation to get over vmalloc
> faults. At that time, NMIs couldn't handle faults and many callchains were
> populated in NMIs. We had serious crashes because of per cpu memory faults.

Is there seriously more than 512GB of per-cpu virtual space or
whatever's needed to exceed a single pgd on x86_64?

And there are definitely placed that access per-cpu data in contexts
in which a non-IST fault is not allowed.  Maybe not dynamic per-cpu
data, though.

--Andy

>
> I think that lazy adressing is there for allocation performance reasons. But
> still having faultable per cpu memory is insame IMHO.
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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