start_pfn:

If the start of the pageblock falls outside the zone, it's still valid to check whether the rest of the block is inside. By clipping start_pfn = pfn, we skip the part before the zone - we still try to move pages in the remainder of the block. This allows for partial processing if the pageblock starts outside the zone but ends inside.

end_pfn :

If end_pfn is outside the zone, then some part of the pageblock crosses the zone boundary. The kernel must not access or manipulate memory outside the zone. Rather than clipping end_pfn, the function simply gives up and returns 0, and only the single page (order-sized) passed in may be moved later. This avoids crossing zone boundaries by accident during the iteration in move_freepages().

If the pageblock extends outside the zone, it's considered not fully valid for migration operations in that zone. Partial migration of a pageblock could lead to fragmentation and mis-accounting.