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

[Davidlohr Bueso <dave@stgolabs.net>]

On Fri, 25 Oct 2024, 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: Davidlohr Bueso <dave@stgolabs.net>