On 12/6/2012 2:12 AM, Mel Gorman wrote:
On Tue, Dec 04, 2012 at 02:10:01PM -0800, Laura Abbott wrote: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. The correct fix is to round down zone_start_pfn for the bit index calculation. Rather than do this calculation everytime, store this precalcualted algined start in the zone structure to allow the actual start_pfn to be used elsewhere. Change-Id: I13e2f53f50db294f38ec86138c17c6fe29f0ee82 Signed-off-by: Laura Abbott <lauraa@xxxxxxxxxxxxxx>Hi Laura, There should be no need to add a new zone field. It's probably ok in terms of functionality but it does mean that we have to worry about things like hotplug (FWIW, should be fine) and the memory overhead is added even on CONFIG_SPARSEMEM where it is not needed. Instead, mask out the lower bits in pfn_to_bitidx() using the same round_down trick you already do. The cost is negligible. Thanks.
I was debating if storing the size was actually necessary. I'll resubmit with the calculation done directly in the function.
Thanks, Laura -- Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>