On 1/28/21 2:04 PM, 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>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---
> 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) {
> /*
> @@ -1767,12 +1772,8 @@ 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);
> @@ -1780,15 +1781,10 @@ 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);
> }
> @@ -1799,9 +1795,10 @@ 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;
> }
>
>
