Re: [PATCH mm-unstable v3 2/2] mm/vmscan: apply too_many_isolated() throttling to MGLRU eviction

On Tue, Aug 18, 2026 at 8:49 PM Hui Zhu <[email protected]> wrote:
>
> From: Hui Zhu <[email protected]>
>
> The legacy path throttles direct reclaim in shrink_inactive_list()
> when too many isolated folios pile up, but MGLRU's evict_folios()
> isolates folios without this check, which can lead to unnecessary
> swapping, thrashing and OOM.
>
> With the NR_ISOLATED counters now updated in evict_folios(), extract
> the throttling loop from shrink_inactive_list() into
> throttle_is_throttled() and reuse it in evict_folios(). Since the
> type to isolate is unknown until isolation and isolate_folios() may
> fall back to the other type, check all evictable types with
> for_each_evictable_type() and throttle if any of them has too many
> isolated folios.
>

I feel this is unlikely to work. MGLRU behaves quite differently from the
active/inactive LRU, and its `NR_INACTIVE_FILE` accounting is also very
different.

With the active/inactive LRU, shrink_inactive_list() ensures that we
always have an inactive list with pages available for reclaim. With MGLRU,
however, a generation can legitimately point to an empty list, so this
assumption does not hold.

try_to_inc_min_seq:

        /* see the comment on lru_gen_folio */
        if (swappiness && swappiness <= MAX_SWAPPINESS) {
                unsigned long seq = lrugen->max_seq - MIN_NR_GENS;

                if (min_seq[LRU_GEN_ANON] > seq && min_seq[LRU_GEN_FILE] < seq)
                        min_seq[LRU_GEN_ANON] = seq;
                else if (min_seq[LRU_GEN_FILE] > seq &&
min_seq[LRU_GEN_ANON] < seq)
                        min_seq[LRU_GEN_FILE] = seq;
        }

At that point, we have no inactive pages for the type, so the
throttle will take effect when the following condition is true:

too_many = isolated > inactive;

With MGLRU, however, we can still fall back to the other type even
when there are no inactive pages for the current type.

BTW, if we are hitting isolated > inactive with MGLRU, it probably
means the generations are quite imbalanced—we are running out of
reclaimable generations. In that case, we may actually want
reclamation to proceed with aging instead.

> If a fatal signal is pending, fake reclaim progress the same way the
> legacy path does, so the dying task exits reclaim quickly instead of
> being held in the throttle.
>
> Signed-off-by: Hui Zhu <[email protected]>
> ---
>  mm/vmscan.c | 71 ++++++++++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 59 insertions(+), 12 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 98226bb021f3..6fe8824430ac 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1819,7 +1819,7 @@ bool folio_isolate_lru(struct folio *folio)
>   * the LRU list will go small and be scanned faster than necessary, leading to
>   * unnecessary swapping, thrashing and OOM.
>   */
> -static bool too_many_isolated(struct pglist_data *pgdat, int file,
> +static bool too_many_isolated(struct pglist_data *pgdat, bool file,
>                 struct scan_control *sc)
>  {
>         unsigned long inactive, isolated;
> @@ -1856,6 +1856,37 @@ static bool too_many_isolated(struct pglist_data *pgdat, int file,
>         return too_many;
>  }
>
> +/*
> + * Throttle reclaim if too many isolated folios are piling up. If this makes
> + * no progress, the caller is probably looping on unevictable folios, so give
> + * up. Returns true to tell the caller to stop reclaiming, and sets @fatal
> + * if the task received a fatal signal while waiting, so that the caller can
> + * bail out faster.
> + */
> +static bool throttle_is_throttled(struct pglist_data *pgdat, bool file,
> +                                 struct scan_control *sc, bool *fatal)
> +{
> +       bool stalled = false;
> +
> +       *fatal = false;

TBH, I find the name quite weird :-)

Best Regards
Barry