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

From: Mel Gorman
Date: Mon Feb 18 2013 - 09:46:34 EST

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
> >> CONFIG_SPARSEMEM defined.
> >
> > 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
> corruption).

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>

Mel Gorman
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