[PATCH] mm: Ensure that pfn_valid is called once per pageblock whenreserving pageblocks

[Michal Hocko]

Hi,
the patch bellow fixes a crash during boot (when we set up reserved
page blocks) if zone start_pfn is not block aligned. The issue has
been introduced in 3.0-rc1 by 6d3163ce: mm: check if any page in a
pageblock is reserved before marking it MIGRATE_RESERVE.

I think this is 3.2 and stable material.
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