Re: frequent lockups in 3.18rc4

From: Frederic Weisbecker
Date: Wed Nov 19 2014 - 17:56:25 EST

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.

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

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