Re: [RFC PATCH v2] mm: Improve pgdat_balanced() to avoid over-reclamation for higher-order allocation

[Johannes Weiner]

Hi Barry,

On Wed, Apr 22, 2026 at 10:18:42AM +0800, Barry Song (Xiaomi) wrote:
> We may encounter cases where the system still has plenty of free
> memory, but cannot satisfy higher-order allocations. On phones, we
> have observed that bursty network transfers can cause devices to
> heat up. Baolin and Kairui have seen similar behavior on servers.
> 
> Currently, kswapd behaves as follows: when a higher-order allocation
> is issued with __GFP_KSWAPD_RECLAIM, pgdat_balanced() returns false
> because __zone_watermark_ok() fails if no suitable higher-order
> pages exist, even when free memory is well above the high watermark.
> As a result, kswapd_shrink_node() sets an excessively large
> sc->nr_to_reclaim and attempts aggressive reclamation:
> 
>  for_each_managed_zone_pgdat(zone, pgdat, z, sc->reclaim_idx) {
>  	sc->nr_to_reclaim += max(high_wmark_pages(zone), SWAP_CLUSTER_MAX);
>  }
> 
> We have an opportunity to re-evaluate the balance by resetting
> sc->order to 0 after shrink_node() with the following code
> in kswapd_shrink_node():
>  /*
>   * Fragmentation may mean that the system cannot be rebalanced for
>   * high-order allocations. If twice the allocation size has been
>   * reclaimed then recheck watermarks only at order-0 to prevent
>   * excessive reclaim.
>   */
>  if (sc->order && sc->nr_reclaimed >= compact_gap(sc->order))
>  	sc->order = 0;
> 
> But we have actually scanned and over-reclaimed far more than
> compact_gap(sc->order).

Do you have traces for how much it overshoots?

> If higher-order allocations continue, we may see persistently high
> kswapd CPU utilization coexisting with plenty of free memory in the
> system.
> 
> We may want to evaluate the situation earlier at the beginning.
> If there is plenty of free memory, we could avoid triggering
> reclamation with an excessively large sc->nr_to_reclaim value
> and instead prefer compaction.
> 
> Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> Cc: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> Cc: Lorenzo Stoakes <ljs@xxxxxxxxxx>
> Cc: Kairui Song <kasong@xxxxxxxxxxx>
> Cc: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
> Cc: Yuanchu Xie <yuanchu@xxxxxxxxxx>
> Cc: Wei Xu <weixugc@xxxxxxxxxx>
> Co-developed-by: Wang Lian <wanglian@xxxxxxxxxx>
> Co-developed-by: Kunwu Chan <chentao@xxxxxxxxxx>
> Signed-off-by: Barry Song (Xiaomi) <baohua@xxxxxxxxxx>
> ---
>  -RFC v1 was "mm: net: disable kswapd for high-order network
>   buffer allocation":
>  https://lore.kernel.org/linux-mm/20251013101636.69220-1-21cnbao@xxxxxxxxx/
> 
>  mm/vmscan.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index bd1b1aa12581..4f9668aa8eef 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -6964,6 +6964,13 @@ static bool pgdat_balanced(pg_data_t *pgdat, int order, int highest_zoneidx)
>  		if (__zone_watermark_ok(zone, order, mark, highest_zoneidx,
>  					0, free_pages))
>  			return true;
> +		/*
> +		 * Free pages may be well above the watermark, but if
> +		 * higher-order pages are unavailable, kswapd may still
> +		 * trigger excessive reclamation.
> +		 */
> +		if (order && compaction_suitable(zone, order, mark, highest_zoneidx))
> +			return true;

I've tried this in the past, but it was regressing huge page requests
under memory pressure and with higher levels of concurrency:

https://lore.kernel.org/linux-mm/20250411182156.GE366747@xxxxxxxxxxx/

The compaction gap is sized for a single allocation, but
kswapd/kcompactd are a shared resource for potentially hundreds or
thousands of incoming requests. So if there is high demand for
contiguous memory this isn't enough - kswapd gives up too early,
kcompactd efficiency drops, you get storms of direct
reclaim/compaction, and still poor allocation success rates. Continued
kswapd wakeups mean that there is ongoing unsatisfied demand. The
system has to keep moving forward.

That said, it's well possible that we're overshooting that progress
buffer due to running reclaim scans with a high order. It might be a
better idea to look into that?