On 1/16/19 3:33 PM, Mel Gorman wrote: >>> + break; >>> + } >>> + >>> + /* >>> + * If low PFNs are being found and discarded then >>> + * limit the scan as fast searching is finding >>> + * poor candidates. >>> + */ >> >> I wonder about the "low PFNs are being found and discarded" part. Maybe >> I'm missing it, but I don't see them being discarded above, this seems >> to be the first check against cc->migrate_pfn. With the min() part in >> update_fast_start_pfn(), does it mean we can actually go back and rescan >> (or skip thanks to skip bits, anyway) again pageblocks that we already >> scanned? >> > > Extremely poor phrasing. My mind was thinking in terms of discarding > unsuitable candidates as they were below the migration scanner and it > did not translate properly. > > Based on your feedback, how does the following untested diff look? IMHO better. Meanwhile I noticed that the next patch removes the set_pageblock_skip() so maybe it's needless churn to introduce the fast_find_block, but I'll check more closely. The new comment about pfns below cc->migrate_pfn is better but I still wonder if it would be better to really skip over those candidates (they are still called unsuitable) and not go backwards with cc->migrate_pfn. But if you think the pageblock skip bits and halving of limit minimizes pointless rescan sufficiently, then fine.
