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mm: use aligned zone start for pfn_to_bitidx calculation
The current calculation in pfn_to_bitidx assumes that (pfn - zone->zone_start_pfn) >> pageblock_order will return the same bit for all pfn in a pageblock. If zone_start_pfn is not aligned to pageblock_nr_pages, this may not always be correct. Consider the following with pageblock order = 10, zone start 2MB: pfn | pfn - zone start | (pfn - zone start) >> page block order ---------------------------------------------------------------- 0x26000 | 0x25e00 | 0x97 0x26100 | 0x25f00 | 0x97 0x26200 | 0x26000 | 0x98 0x26300 | 0x26100 | 0x98 This means that calling {get,set}_pageblock_migratetype on a single page will not set the migratetype for the full block. Fix this by rounding down zone_start_pfn when doing the bitidx calculation. For our use case, the effects of this bug were mostly tied to the fact that CMA allocations would either take a long time or fail to happen. Depending on the driver using CMA, this could result in anything from visual glitches to application failures. Signed-off-by: Laura Abbott <lauraa@codeaurora.org> Acked-by: Mel Gorman <mgorman@suse.de> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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@ -5604,7 +5604,7 @@ static inline int pfn_to_bitidx(struct zone *zone, unsigned long pfn)
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pfn &= (PAGES_PER_SECTION-1);
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return (pfn >> pageblock_order) * NR_PAGEBLOCK_BITS;
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#else
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pfn = pfn - zone->zone_start_pfn;
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pfn = pfn - round_down(zone->zone_start_pfn, pageblock_nr_pages);
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return (pfn >> pageblock_order) * NR_PAGEBLOCK_BITS;
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#endif /* CONFIG_SPARSEMEM */
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}
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