On Fri, Oct 25, 2024 at 10:17:24AM -0400, 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@xxxxxxxxxx>
> Fixes: 26aa2d199d6f2 ("mm/migrate: demote pages during reclaim")
> Cc: stable@xxxxxxxxxxxxxxx
Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>
--
Oscar Salvador
SUSE Labs
