linux-kernel - Re: frequent lockups in 3.18rc4

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Date:	Thu, 20 Nov 2014 00:09:22 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Frederic Weisbecker <fweisbec@...il.com>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>, Don Zickus <dzickus@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...capital.net>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
Subject: Re: frequent lockups in 3.18rc4

On Wed, 19 Nov 2014, Frederic Weisbecker 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.

Sorry, I can't follow your argumentation here.

pcpu_alloc()
   ....
area_found:
   ....

        /* clear the areas and return address relative to base address */
        for_each_possible_cpu(cpu)
                memset((void *)pcpu_chunk_addr(chunk, cpu, 0) + off, 0, size);

How would that memset fail to establish the mapping, which is
btw. already established via:

     pcpu_populate_chunk()
  
already before that memset?   	    
 
Are we talking about different per cpu allocators here or am I missing
something completely non obvious?

Thanks,

	tglx
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