Re: [PATCH] vmscan,migrate: fix double-decrement on node stats when demoting pages

[Shakeel Butt <shakeel.butt@linux.dev>]

On Fri, Oct 25, 2024 at 10:17:24AM GMT, Gregory Price wrote:
> When numa balancing is enabled with demotion, vmscan will call
> migrate_pages when shrinking LRUs.  Successful demotions will
> cause node vmstat numbers to double-decrement, leading to an
> imbalanced page count.  The result is dmesg output like such:
> 
> $ cat /proc/sys/vm/stat_refresh
> 
> [77383.088417] vmstat_refresh: nr_isolated_anon -103212
> [77383.088417] vmstat_refresh: nr_isolated_file -899642
> 
> This negative value may impact compaction and reclaim throttling.
> 
> The double-decrement occurs in the migrate_pages path:
> 
> caller to shrink_folio_list decrements the count
>   shrink_folio_list
>     demote_folio_list
>       migrate_pages
>         migrate_pages_batch
>           migrate_folio_move
>             migrate_folio_done
>               mod_node_page_state(-ve) <- second decrement
> 
> This path happens for SUCCESSFUL migrations, not failures. Typically
> callers to migrate_pages are required to handle putback/accounting for
> failures, but this is already handled in the shrink code.
> 
> When accounting for migrations, instead do not decrement the count
> when the migration reason is MR_DEMOTION. As of v6.11, this demotion
> logic is the only source of MR_DEMOTION.
> 
> Signed-off-by: Gregory Price <gourry@gourry.net>
> Fixes: 26aa2d199d6f2 ("mm/migrate: demote pages during reclaim")
> Cc: stable@vger.kernel.org

Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>

This patch looks good for stable backports. For future I wonder if
instead of migrate_pages(), the caller providing the isolated folios,
manages the isolated stats (increments and decrements) similar to how
reclaim does it.

> ---
>  mm/migrate.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 923ea80ba744..e3aac274cf16 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1099,7 +1099,7 @@ static void migrate_folio_done(struct folio *src,
>  	 * not accounted to NR_ISOLATED_*. They can be recognized
>  	 * as __folio_test_movable
>  	 */
> -	if (likely(!__folio_test_movable(src)))
> +	if (likely(!__folio_test_movable(src)) && reason != MR_DEMOTION)
>  		mod_node_page_state(folio_pgdat(src), NR_ISOLATED_ANON +
>  				    folio_is_file_lru(src), -folio_nr_pages(src));
>  
> -- 
> 2.43.0
>