mm/compaction: fix 'limit' in fast_isolate_freepages

Because of 'min(1, ...)', fast_isolate_freepages set 'limit' to 0 or 1.
This takes away the opportunities of find candinate pages.  So, by making
enough scans available, increases the probability of finding the
appropriate freepage.

Tested it on the thpscale and the results are as follows.

                                        5.12.0                 5.12.0
                                      valnilla                patched
Amean     fault-both-1       598.15 (   0.00%)      592.56 (   0.93%)
Amean     fault-both-3      1494.47 (   0.00%)     1514.35 (  -1.33%)
Amean     fault-both-5      2519.48 (   0.00%)     2471.76 (   1.89%)
Amean     fault-both-7      3173.85 (   0.00%)     3079.19 (   2.98%)
Amean     fault-both-12     8063.83 (   0.00%)     7858.29 (   2.55%)
Amean     fault-both-18     8781.20 (   0.00%)     7827.70 *  10.86%*
Amean     fault-both-24    12576.44 (   0.00%)    12250.20 (   2.59%)
Amean     fault-both-30    18503.27 (   0.00%)    17528.11 *   5.27%*
Amean     fault-both-32    16133.69 (   0.00%)    13874.24 *  14.00%*

                                           5.12.0         5.12.0
                                          vanilla        patched
Ops Compaction migrate scanned         6547133.00     5963901.00
Ops Compaction free scanned           32452453.00    26609101.00

                        5.12        5.12
                     vanilla     patched
Duration User          27.99       28.84
Duration System       244.08      236.76
Duration Elapsed       78.27       78.38

Link: https://lkml.kernel.org/r/20210626082443.22547-1-vvghjk1234@gmail.com
Fixes: 5a811889 ("mm, compaction: use free lists to quickly locate a migration target")
Signed-off-by: Wonhyuk Yang <vvghjk1234@gmail.com>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/compaction.c

@@ -1378,7 +1378,7 @@ static int next_search_order(struct compact_control *cc, int order)
 static unsigned long
 fast_isolate_freepages(struct compact_control *cc)
 {
-	unsigned int limit = min(1U, freelist_scan_limit(cc) >> 1);
+	unsigned int limit = max(1U, freelist_scan_limit(cc) >> 1);
 	unsigned int nr_scanned = 0;
 	unsigned long low_pfn, min_pfn, highest = 0;
 	unsigned long nr_isolated = 0;
@@ -1490,11 +1490,11 @@ fast_isolate_freepages(struct compact_control *cc)
 		spin_unlock_irqrestore(&cc->zone->lock, flags);
 
 		/*
-		 * Smaller scan on next order so the total scan ig related
+		 * Smaller scan on next order so the total scan is related
 		 * to freelist_scan_limit.
 		 */
 		if (order_scanned >= limit)
-			limit = min(1U, limit >> 1);
+			limit = max(1U, limit >> 1);
 	}
 
 	if (!page) {