On Thu, Dec 1, 2022 at 3:33 PM Mina Almasry <almasrymina@xxxxxxxxxx> wrote:
>
> 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.
Makes sense to me. Reviewed-by: Yang Shi <shy828301@xxxxxxxxx>
>
> Signed-off-by: Mina Almasry <almasrymina@xxxxxxxxxx>
> ---
> 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.
> + */
> 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
>
