Re: [PATCH mmotm 3/3] mm, compaction: prevent nr_isolated_* from going negative

[Joonsoo Kim]

On Mon, Apr 25, 2016 at 03:35:50PM +0200, Vlastimil Babka wrote:
> From: Hugh Dickins <hughd@xxxxxxxxxx>
> 
> /proc/sys/vm/stat_refresh warns nr_isolated_anon and nr_isolated_file
> go increasingly negative under compaction: which would add delay when
> should be none, or no delay when should delay.  putback_movable_pages()
> decrements the NR_ISOLATED counts which acct_isolated() increments,
> so isolate_migratepages_block() needs to acct before putback in that
> special case. It's also useful to reset cc->nr_migratepages after putback
> so we don't needlessly return too early on the COMPACT_CLUSTER_MAX check.
> 
> Also it's easier to track the life of cc->migratepages if we don't assign
> it to a migratelist variable.
> 
> Fixes: mmotm mm-compaction-skip-blocks-where-isolation-fails-in-async-direct-compaction.patch
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---
>  mm/compaction.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 6a49d1b35515..78552009e6ec 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -638,7 +638,6 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>  {
>  	struct zone *zone = cc->zone;
>  	unsigned long nr_scanned = 0, nr_isolated = 0;
> -	struct list_head *migratelist = &cc->migratepages;
>  	struct lruvec *lruvec;
>  	unsigned long flags = 0;
>  	bool locked = false;
> @@ -812,7 +811,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>  		del_page_from_lru_list(page, lruvec, page_lru(page));
>  
>  isolate_success:
> -		list_add(&page->lru, migratelist);
> +		list_add(&page->lru, &cc->migratepages);
>  		cc->nr_migratepages++;
>  		nr_isolated++;
>  
> @@ -846,9 +845,11 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>  				spin_unlock_irqrestore(&zone->lru_lock,	flags);
>  				locked = false;
>  			}
> -			putback_movable_pages(migratelist);
> -			nr_isolated = 0;
> +			acct_isolated(zone, cc);
> +			putback_movable_pages(&cc->migratepages);
> +			cc->nr_migratepages = 0;
>  			cc->last_migrated_pfn = 0;
> +			nr_isolated = 0;

Is it better to use separate list and merge it cc->migratepages when
finishing instead of using cc->migratepages directly? If
isolate_migratepages() try to isolate more than one page block and keep
isolated page on previous pageblock, this putback all will invalidate
all the previous work. It would be beyond of the scope of this
function. Now, isolate_migratepages() try to isolate the page in one
pageblock so this code is safe. But, I think that removing such
dependency will be helpful in the future. I'm not strongly insisting it
so if you think it's not useful thing, please ignore this comment.

Thanks.