mm/page_alloc: drain the requested list first during bulk free

Prior to the series, pindex 0 (order-0 MIGRATE_UNMOVABLE) was always
skipped first and the precise reason is forgotten.  A potential reason
may have been to artificially preserve MIGRATE_UNMOVABLE but there is no
reason why that would be optimal as it depends on the workload.  The
more likely reason is that it was less complicated to do a pre-increment
instead of a post-increment in terms of overall code flow.  As
free_pcppages_bulk() now typically receives the pindex of the PCP list
that exceeded high, always start draining that list.

Link: https://lkml.kernel.org/r/20220217002227.5739-5-mgorman@techsingularity.net
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Tested-by: Aaron Lu <aaron.lu@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Jesper Dangaard Brouer <brouer@redhat.com>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -1460,6 +1460,10 @@ static void free_pcppages_bulk(struct zone *zone, int count,
 	 * below while (list_empty(list)) loop.
 	 */
 	count = min(pcp->count, count);
+
+	/* Ensure requested pindex is drained first. */
+	pindex = pindex - 1;
+
 	while (count > 0) {
 		struct list_head *list;
 		int nr_pages;