Re: [PATCH v2] mm/compactoin: Fix misbehaviors of fast_find_migrateblock()

[Mel Gorman <mgorman@techsingularity.net>]

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@gmail.com>
> ---
> 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@gmail.com
> ---
>  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