Re: [-rc7 regression] Buggy commit: "mm: use aligned zone start forpfn_to_bitidx calculation"

From: Laura Abbott
Date: Mon Feb 18 2013 - 13:42:14 EST

On 2/18/2013 6:46 AM, Mel Gorman wrote:
On Sat, Feb 16, 2013 at 10:26:30AM -0800, Linus Torvalds wrote:
On Fri, Feb 15, 2013 at 3:44 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:

c060f943d092 may be related as you config does not have

Right, that's the commit causing the x86 regression:

c060f943d0929f3e429c5d9522290584f6281d6e is the first bad commit
commit c060f943d0929f3e429c5d9522290584f6281d6e
Date: Fri Jan 11 14:31:51 2013 -0800

mm: use aligned zone start for pfn_to_bitidx calculation

Ok, looking more at this, I don't really want to revert it, and I have
an idea of what is wrong.

When we allocate the zone use bitmap, we do not take the
zone_start_pfn into account. So I *think* that what happens is that
"pfn_to_bitidx()" simply overruns the allocation for unaligned zonesm
and the spinlock just happens to be right after (or the overrun causes
some other memory corruption that then indirectly causes the spinlock

More likely the latter. I'd expect the usemap to be adjacent to the
zone->wait_table because of when they are allocated by the bootmem
allocator. This would break wait_on_page_[locked|writeback] at the very
least. If page_waitqueue() returned a corrupt pointer from the wait table
then it would lead to further corruption elsewhere each time wait_on_page_foo
was called.

So I'm wondering if the fix is simply something like the attached
patch. It takes the zone_start_pfn into account when allocating the
zone bitmap.

Laura? Mel?

Looks correct to me and should cc stable@xxxxxxxxxxxxxxx

Acked-by: Mel Gorman <mgorman@xxxxxxx>

I had convinced myself when I sent the patch that everything would just shift down and there wouldn't need to be an array size increase. Looks like my math was bogus and I'll double check it next time. The updated version looks okay to me and I'll pull in the patch for more testing on the setup that originally found the problem this week.


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