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Re: system lockup with 2.6.29 on Cavium/Octeon

To: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Subject: Re: system lockup with 2.6.29 on Cavium/Octeon
From: Greg Ungerer <gerg@snapgear.com>
Date: Fri, 22 May 2009 11:19:00 +1000
Cc: ralf@linux-mips.org, linux-mips@linux-mips.org
In-reply-to: <20090521.235020.173372074.anemo@mba.ocn.ne.jp>
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References: <4A139F50.7050409@snapgear.com> <20090520142604.GA29677@linux-mips.org> <20090521.235020.173372074.anemo@mba.ocn.ne.jp>
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Hi Atsushi,

Atsushi Nemoto wrote:
On Wed, 20 May 2009 15:26:04 +0100, Ralf Baechle <ralf@linux-mips.org> wrote:
Now the vmalloc area starts at 0xc000000000000000 and the kernel code
and data is all at 0xffffffff80000000 and above. I don't know if the
start and end are reasonable values, but I can see some logic as to
where they come from. The code path that leads to this is via
__vunmap() and __purge_vmap_area_lazy(). So it is not too difficult
to see how we end up with values like this.
Either start or end address is sensible but not the combination - both
addresses should be in the same segment.  Start is in XKSEG, end in CKSEG2
and in between there are vast wastelands of unused address space exabytes
in size.

But the size calculation above with these types of values will result
in still a large number. Larger than the 32bit "int" that is "size".
I see large negative values fall out as size, and so the following
tlbsize check becomes true, and the code spins inside the loop inside
that if statement for a _very_ long time trying to flush tlb entries.

This is of course easily fixed, by making that size "unsigned long".
The patch below trivially does this.

But is this analysis correct?
Yes - but I think we have two issues here.  The one is the calculation
overflowing int for the arguments you're seeing.  The other being that
the arguments simply are looking wrong.

The wrong combination comes from lazy vunmapping which was introduced
in 2.6.28 cycle.  Maybe we can add new API (non-lazy version of
vfree()) to vmalloc.c to implement module_free(), but I suppose
fallbacking to local_flush_tlb_all() in local_flush_tlb_kernel_range()
is enough().

Is there any performance impact on falling back to that?

The flushing due to lazy vunmapping didn't seem to happen
often in the tests I was running.

Regards
Greg


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Greg Ungerer  --  Principal Engineer        EMAIL:     gerg@snapgear.com
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