Re: [PATCH] mm/vmscan: clear hopeless kswapd when global direct reclaim makes progress

[Altan Hacigumus]

On Tue, Jul 14, 2026 at 4:34 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Thu 09-07-26 19:44:29, Altan Hacigumus wrote:
> > Direct reclaim clears the hopeless kswapd state only once the node
> > becomes balanced. This avoids cgroup memory.high reclaim repeatedly
> > reviving a kswapd that cannot balance the node.
> >
> > However, it also prevents global direct reclaim from reviving kswapd.
> > Under sustained memory pressure, global direct reclaim may continue
> > making progress without the node ever reaching the high watermark,
> > leaving reclaim to allocating tasks.
> >
> > Unlike memcg reclaim, global direct reclaim follows the same node-wide
> > reclaim path as kswapd. Clear the hopeless state when global direct
> > reclaim makes progress, while continuing to require a balanced node for
> > memcg reclaim. kswapd_try_clear_hopeless() becomes static since struct
> > scan_control is private to vmscan.c.
> >
> > On a workload with most memory mlocked and the remainder under sustained
> > churn, with swap enabled, comparing the same 60s window:
> >                               base       patched
> >   allocstall (all zones)    413441          6532
> >   pgsteal_direct          15889552        255619
> >   pgsteal_kswapd                 0      26079382
> >   PSI memory full avg60     13.10%         9.37%
> >
> > Direct reclaim was already reclaiming many pages in the baseline;
> > this change lets kswapd resume doing that work asynchronously.
> >
> > Signed-off-by: Altan Hacigumus <ahacigu.linux@xxxxxxxxx>
> > ---
> >  include/linux/mmzone.h |  2 --
> >  mm/vmscan.c            | 23 ++++++++++++++---------
> >  2 files changed, 14 insertions(+), 11 deletions(-)
> >
> > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > index ca2712187147..1db6f8dac927 100644
> > --- a/include/linux/mmzone.h
> > +++ b/include/linux/mmzone.h
> > @@ -1630,8 +1630,6 @@ enum kswapd_clear_hopeless_reason {
> >
> >  void wakeup_kswapd(struct zone *zone, gfp_t gfp_mask, int order,
> >                  enum zone_type highest_zoneidx);
> > -void kswapd_try_clear_hopeless(struct pglist_data *pgdat,
> > -                            unsigned int order, int highest_zoneidx);
> >  void kswapd_clear_hopeless(pg_data_t *pgdat, enum kswapd_clear_hopeless_reason reason);
> >  bool kswapd_test_hopeless(pg_data_t *pgdat);
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 35c3bb15ae96..78fd35a11eee 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -198,6 +198,9 @@ struct scan_control {
> >   */
> >  int vm_swappiness = 60;
> >
> > +static void kswapd_try_clear_hopeless(struct pglist_data *pgdat,
> > +                                   struct scan_control *sc);
> > +
> >  #ifdef CONFIG_MEMCG
> >
> >  /* Returns true for reclaim through cgroup limits or cgroup interfaces. */
> > @@ -5172,7 +5175,7 @@ static void lru_gen_shrink_node(struct pglist_data *pgdat, struct scan_control *
> >       blk_finish_plug(&plug);
> >  done:
> >       if (sc->nr_reclaimed > reclaimed)
> > -             kswapd_try_clear_hopeless(pgdat, sc->order, sc->reclaim_idx);
> > +             kswapd_try_clear_hopeless(pgdat, sc);
> >  }
> >
> >  /******************************************************************************
> > @@ -6251,7 +6254,7 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
> >        * successful direct reclaim run will revive a dormant kswapd.
> >        */
> >       if (reclaimable)
> > -             kswapd_try_clear_hopeless(pgdat, sc->order, sc->reclaim_idx);
> > +             kswapd_try_clear_hopeless(pgdat, sc);
> >       else if (sc->cache_trim_mode)
> >               sc->cache_trim_mode_failed = 1;
> >  }
> > @@ -7530,15 +7533,17 @@ void kswapd_clear_hopeless(pg_data_t *pgdat, enum kswapd_clear_hopeless_reason r
> >  }
> >
> >  /*
> > - * Reset kswapd_failures only when the node is balanced. Without this
> > - * check, successful direct reclaim (e.g., from cgroup memory.high
> > - * throttling) can keep resetting kswapd_failures even when the node
> > - * cannot be balanced, causing kswapd to run endlessly.
> > + * Reset kswapd_failures when the node is balanced, or when global
> > + * direct reclaim makes progress - then kswapd can make progress too.
> > + * Memcg reclaim can succeed where kswapd cannot (memcg protection is
> > + * not enforced against the reclaim target), so its progress resets
> > + * kswapd_failures only when the node is balanced.
> >   */
> > -void kswapd_try_clear_hopeless(struct pglist_data *pgdat,
> > -                            unsigned int order, int highest_zoneidx)
> > +static void kswapd_try_clear_hopeless(struct pglist_data *pgdat,
> > +                                   struct scan_control *sc)
> >  {
> > -     if (pgdat_balanced(pgdat, order, highest_zoneidx))
> > +     if ((!current_is_kswapd() && !cgroup_reclaim(sc)) ||
> > +         pgdat_balanced(pgdat, sc->order, sc->reclaim_idx))
> >               kswapd_clear_hopeless(pgdat, current_is_kswapd() ?
> >                       KSWAPD_CLEAR_HOPELESS_KSWAPD : KSWAPD_CLEAR_HOPELESS_DIRECT);
>
> Is there any particular reason why pgdat_balanced is evaluated for
> cgroup_reclaim? Shouldn't this be global reclaim logic only?
> I.e.
>         if (cgroup_reclaim(sc))
>                 return;
>         if (!current_is_kswapd() || pgdat_balanced(pgdat, sc->order, sc->reclaim_idx))
>                 kswapd_clear_hopeless(pgdat, current_is_kswapd() ? KSWAPD_CLEAR_HOPELESS_KSWAPD : KSWAPD_CLEAR_HOPELESS_DIRECT);

[1] Only to keep the diff to one behavior change. Agreed, the early
return is stricter and cleaner, and reason=DIRECT then means strictly
global direct reclaim.

[2] Also, there is one more condition I've been looking at, from the
Sashiko review (a theoretical corner): skip the reset while tasks are
throttled on pfmemalloc_wait, so that it cannot defer the hopeless
escape hatch in allow_direct_reclaim().


[1] and [2] combined, it would look like:
static void kswapd_try_clear_hopeless(struct pglist_data *pgdat,
      struct scan_control *sc)
{
if (cgroup_reclaim(sc))
return;

/*
* Don't reset while tasks are throttled on pfmemalloc_wait -
* kswapd turning hopeless is their escape hatch.
*/
if ((!waitqueue_active(&pgdat->pfmemalloc_wait) &&
     !current_is_kswapd()) ||
    pgdat_balanced(pgdat, sc->order, sc->reclaim_idx))
kswapd_clear_hopeless(pgdat, current_is_kswapd() ?
KSWAPD_CLEAR_HOPELESS_KSWAPD : KSWAPD_CLEAR_HOPELESS_DIRECT);
}


>
>
> >  }
> > --
> > 2.55.0
>
> --
> Michal Hocko
> SUSE Labs

Thanks,
  Altan