Commit 15d28d0d authored by Wonhyuk Yang's avatar Wonhyuk Yang Committed by Linus Torvalds
Browse files

mm/compaction: fix misbehaviors of fast_find_migrateblock() 

In the fast_find_migrateblock(), it iterates ocer the freelist to find the
proper pageblock.  But there are some misbehaviors.

First, if the page we found is equal to cc->migrate_pfn, it is considered
that we didn't find a suitable pageblock.  Secondly, if the loop was
terminated because order is less than PAGE_ALLOC_COSTLY_ORDER, it could be
considered that we found a suitable one.  Thirdly, if the skip bit is set
on the page block and we goto continue, it doesn't check nr_scanned.
Fourthly, if the page block's skip bit is set, it checks that page block
is the last of list, which is unnecessary.

Link: https://lkml.kernel.org/r/20210128130411.6125-1-vvghjk1234@gmail.com


Fixes: 70b44595 ("mm, compaction: use free lists to quickly locate a migration source")
Signed-off-by: default avatarWonhyuk Yang <vvghjk1234@gmail.com>
Acked-by: default avatarVlastimil Babka <vbabka@suse.cz>
Cc: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 40d7e203

mm/compaction.c

+12 −15
Original line number Diff line number Diff line
@@ -1698,6 +1698,7 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc) 
	unsigned long pfn = cc->migrate_pfn;

	unsigned long high_pfn;

	int order;

	bool found_block = false;



	/* Skip hints are relied on to avoid repeats on the fast search */

	if (cc->ignore_skip_hint)
@@ -1740,7 +1741,7 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc) 
	high_pfn = pageblock_start_pfn(cc->migrate_pfn + distance);



	for (order = cc->order - 1;

	     order >= PAGE_ALLOC_COSTLY_ORDER && pfn == cc->migrate_pfn && nr_scanned < limit;

	     order >= PAGE_ALLOC_COSTLY_ORDER && !found_block && nr_scanned < limit;

	     order--) {

		struct free_area *area = &cc->zone->free_area[order];

		struct list_head *freelist;
@@ -1755,7 +1756,11 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc) 
		list_for_each_entry(freepage, freelist, lru) {

			unsigned long free_pfn;



			nr_scanned++;

			if (nr_scanned++ >= limit) {

				move_freelist_tail(freelist, freepage);

				break;

			}



			free_pfn = page_to_pfn(freepage);

			if (free_pfn < high_pfn) {

				/*
@@ -1764,12 +1769,8 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc) 
				 * the list assumes an entry is deleted, not

				 * reordered.

				 */

				if (get_pageblock_skip(freepage)) {

					if (list_is_last(freelist, &freepage->lru))

						break;



				if (get_pageblock_skip(freepage))

					continue;

				}



				/* Reorder to so a future search skips recent pages */

				move_freelist_tail(freelist, freepage);
@@ -1777,15 +1778,10 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc) 
				update_fast_start_pfn(cc, free_pfn);

				pfn = pageblock_start_pfn(free_pfn);

				cc->fast_search_fail = 0;

				found_block = true;

				set_pageblock_skip(freepage);

				break;

			}



			if (nr_scanned >= limit) {

				cc->fast_search_fail++;

				move_freelist_tail(freelist, freepage);

				break;

			}

		}

		spin_unlock_irqrestore(&cc->zone->lock, flags);

	}
@@ -1796,9 +1792,10 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc) 
	 * If fast scanning failed then use a cached entry for a page block

	 * that had free pages as the basis for starting a linear scan.

	 */

	if (pfn == cc->migrate_pfn)

	if (!found_block) {

		cc->fast_search_fail++;

		pfn = reinit_migrate_pfn(cc);



	}

	return pfn;

}