Re: [PATCH v1] mm: disable top-tier fallback to reclaim on proactive reclaim

["Huang, Ying" <ying.huang@intel.com>]

Mina Almasry <almasrymina@google.com> writes:

> Reclaiming directly from top tier nodes breaks the aging pipeline of
> memory tiers.  If we have a RAM -> CXL -> storage hierarchy, we
> should demote from RAM to CXL and from CXL to storage. If we reclaim
> a page from RAM, it means we 'demote' it directly from RAM to storage,
> bypassing potentially a huge amount of pages colder than it in CXL.
>
> However disabling reclaim from top tier nodes entirely would cause ooms
> in edge scenarios where lower tier memory is unreclaimable for whatever
> reason, e.g. memory being mlocked() or too hot to reclaim.  In these
> cases we would rather the job run with a performance regression rather
> than it oom altogether.
>
> However, we can disable reclaim from top tier nodes for proactive reclaim.
> That reclaim is not real memory pressure, and we don't have any cause to
> be breaking the aging pipeline.
>
> Signed-off-by: Mina Almasry <almasrymina@google.com>
> ---
>  mm/vmscan.c | 27 ++++++++++++++++++++++++---
>  1 file changed, 24 insertions(+), 3 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 23fc5b523764..6eb130e57920 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2088,10 +2088,31 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>  	nr_reclaimed += demote_folio_list(&demote_folios, pgdat);
>  	/* Folios that could not be demoted are still in @demote_folios */
>  	if (!list_empty(&demote_folios)) {
> -		/* Folios which weren't demoted go back on @folio_list for retry: */
> +		/*
> +		 * Folios which weren't demoted go back on @folio_list.
> +		 */

I don't we should change comments style here.  Why not just

+		/* Folios which weren't demoted go back on @folio_list. */

Other than this, the patch LGTM, Thanks!

Reviewed-by: "Huang, Ying" <ying.huang@intel.com>

>  		list_splice_init(&demote_folios, folio_list);
> -		do_demote_pass = false;
> -		goto retry;
> +
> +		/*
> +		 * goto retry to reclaim the undemoted folios in folio_list if
> +		 * desired.
> +		 *
> +		 * Reclaiming directly from top tier nodes is not often desired
> +		 * due to it breaking the LRU ordering: in general memory
> +		 * should be reclaimed from lower tier nodes and demoted from
> +		 * top tier nodes.
> +		 *
> +		 * However, disabling reclaim from top tier nodes entirely
> +		 * would cause ooms in edge scenarios where lower tier memory
> +		 * is unreclaimable for whatever reason, eg memory being
> +		 * mlocked or too hot to reclaim. We can disable reclaim
> +		 * from top tier nodes in proactive reclaim though as that is
> +		 * not real memory pressure.
> +		 */
> +		if (!sc->proactive) {
> +			do_demote_pass = false;
> +			goto retry;
> +		}
>  	}
>
>  	pgactivate = stat->nr_activate[0] + stat->nr_activate[1];
> --
> 2.39.0.rc0.267.gcb52ba06e7-goog