On Thu, Jan 28, 2021 at 10:04:11PM +0900, Wonhyuk Yang wrote:
> In the fast_find_migrateblock(), It iterate freelist to find
> 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 found suitable pageblock. Second,
> if the loop was terminated because order is less than
> PAGE_ALLOC_COSTLY_ORDER, it could be considered that we found
> suitable one. Third, if the skip bit is set on the page block
> and go continue, it doesn't check the nr_scanned. Fourth, if
> the page block's skip bit is set, it check that page block is
> the last of list. But it is unnecessary.
>
> Fixes: 70b44595eafe9 ("mm, compaction: use free lists to quickly locate
> a migration source")
>
> Signed-off-by: Wonhyuk Yang <vvghjk1234@xxxxxxxxx>
> ---
> Changes in v2:
> - Follow Vlastimil's suggestions, using bool varable.
> - Fix new misbehavior and remove unecessary .
>
> v1: https://lore.kernel.org/linux-mm/20210123154320.24278-1-vvghjk1234@xxxxxxxxx
> ---
> mm/compaction.c | 27 ++++++++++++---------------
> 1 file changed, 12 insertions(+), 15 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index e5acb9714436..8e5cd9d20435 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1701,6 +1701,7 @@ 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)
> @@ -1743,7 +1744,7 @@ 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;
> @@ -1758,7 +1759,11 @@ 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) {
> /*
As nr_scanned is post-incremented, this will still consider if the page
should be used when the limit is reached. ++nr_scanned?
--
Mel Gorman
SUSE Labs
