linux-kernel - Re: frequent lockups in 3.18rc4

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Date:	Wed, 19 Nov 2014 16:30:55 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Don Zickus <dzickus@...hat.com>, Dave Jones <davej@...hat.com>,
	"the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: frequent lockups in 3.18rc4

On Nov 19, 2014 4:00 PM, "Thomas Gleixner" <tglx@...utronix.de> wrote:
>
> On Wed, 19 Nov 2014, Andy Lutomirski wrote:
> > On Wed, Nov 19, 2014 at 3:09 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> > > 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?
> >
> > I think that this will map them into init_mm->pgd and
> > current->active_mm->pgd, but it won't necessarily map them into the
> > rest of the pgds.
>
> And why would mapping them into the kernel mapping, i.e. init_mm not
> be sufficient?

Because the kernel can run with any pgd loaded into cr3, and we rely
on vmalloc_fault to lazily populate pgds in all the non-init pgds as
needed.  But this only happens if the first TLB-missing reference to
the pgd in question with any given cr3 value happens from a safe
context.

This is why I think that the grsec kernels will crash on very large
memory systems.  They don't seem to get this right for the kernel
stack, and a page fault trying to access the stack is a big no-no.

--Andy

>
> We are talking about kernel memory and not some random user space
> mapping.
>
> Thanks,
>
>         tglx
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