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

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

[Barry Song (Xiaomi)]

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). 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@linux.alibaba.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Cc: Wei Xu <weixugc@google.com>
Co-developed-by: Wang Lian <wanglian@kylinos.cn>
Co-developed-by: Kunwu Chan <chentao@kylinos.cn>
Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
---
 -RFC v1 was "mm: net: disable kswapd for high-order network
  buffer allocation":
 https://lore.kernel.org/linux-mm/20251013101636.69220-1-21cnbao@gmail.com/

 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;
 	}
 
 	/*
-- 
2.39.3 (Apple Git-146)

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

[Baolin Wang]

On 4/22/26 10:18 AM, 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). 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@linux.alibaba.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: David Hildenbrand <david@kernel.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Qi Zheng <zhengqi.arch@bytedance.com>
> Cc: Shakeel Butt <shakeel.butt@linux.dev>
> Cc: Lorenzo Stoakes <ljs@kernel.org>
> Cc: Kairui Song <kasong@tencent.com>
> Cc: Axel Rasmussen <axelrasmussen@google.com>
> Cc: Yuanchu Xie <yuanchu@google.com>
> Cc: Wei Xu <weixugc@google.com>
> Co-developed-by: Wang Lian <wanglian@kylinos.cn>
> Co-developed-by: Kunwu Chan <chentao@kylinos.cn>
> Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
> ---

Thanks Barry for sending out the RFC patch for discussion.

Yes, we have indeed seen reports from our customers' scenarios where 
fragmentation caused kswapd to be woken up and reclaim too many file 
folios (even when free memory was sufficient), leading to severe I/O 
contention that impacted some applications.

However, I'm concerned that this patch might also have side effects, 
such as affecting system defragmentation. In some scenarios, directly 
reclaiming clean pagecache to free up space might be a faster way to 
defragment. At the very least, I think under defrag_mode, we should be 
more aggressive about defragmentation (including reclaiming some memory 
by kswapd).

>   -RFC v1 was "mm: net: disable kswapd for high-order network
>    buffer allocation":
>   https://lore.kernel.org/linux-mm/20251013101636.69220-1-21cnbao@gmail.com/
> 
>   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;
>   	}
>   
>   	/*

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

[Barry Song]

On Wed, Apr 22, 2026 at 2:59 PM Baolin Wang
<baolin.wang@linux.alibaba.com> wrote:
>
>
>
> On 4/22/26 10:18 AM, 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). 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@linux.alibaba.com>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Cc: David Hildenbrand <david@kernel.org>
> > Cc: Michal Hocko <mhocko@kernel.org>
> > Cc: Qi Zheng <zhengqi.arch@bytedance.com>
> > Cc: Shakeel Butt <shakeel.butt@linux.dev>
> > Cc: Lorenzo Stoakes <ljs@kernel.org>
> > Cc: Kairui Song <kasong@tencent.com>
> > Cc: Axel Rasmussen <axelrasmussen@google.com>
> > Cc: Yuanchu Xie <yuanchu@google.com>
> > Cc: Wei Xu <weixugc@google.com>
> > Co-developed-by: Wang Lian <wanglian@kylinos.cn>
> > Co-developed-by: Kunwu Chan <chentao@kylinos.cn>
> > Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
> > ---
>
> Thanks Barry for sending out the RFC patch for discussion.
>
> Yes, we have indeed seen reports from our customers' scenarios where
> fragmentation caused kswapd to be woken up and reclaim too many file
> folios (even when free memory was sufficient), leading to severe I/O
> contention that impacted some applications.
>
> However, I'm concerned that this patch might also have side effects,
> such as affecting system defragmentation. In some scenarios, directly
> reclaiming clean pagecache to free up space might be a faster way to

balance_pgdat() can still reclaim clean page cache even when
pgdat_balanced() returns true, provided that nr_boost_reclaim is
non-zero.

                /*
                 * If boosting is not active then only reclaim if there are no
                 * eligible zones. Note that sc.reclaim_idx is not used as
                 * buffer_heads_over_limit may have adjusted it.
                 */
                if (!nr_boost_reclaim && balanced)
                        goto out;

                /* Limit the priority of boosting to avoid reclaim writeback */
                if (nr_boost_reclaim && sc.priority == DEF_PRIORITY - 2)
                        raise_priority = false;

                /*
                 * Do not writeback or swap pages for boosted reclaim. The
                 * intent is to relieve pressure not issue sub-optimal IO
                 * from reclaim context. If no pages are reclaimed, the
                 * reclaim will be aborted.
                 */
                sc.may_writepage = !nr_boost_reclaim;
                sc.may_swap = !nr_boost_reclaim;

I find that nr_boost_reclaim is almost always non-zero in bursty
network scenarios. So I guess clean page cache is still reclaimed,
but with much lower kswapd pressure.

> defragment. At the very least, I think under defrag_mode, we should be
> more aggressive about defragmentation (including reclaiming some memory
> by kswapd).

I guess we can keep the current behavior if defrag_mode prefers
over-reclaiming to form contiguous pages. Is it simply an
if (defrag_mode) check?

>
> >   -RFC v1 was "mm: net: disable kswapd for high-order network
> >    buffer allocation":
> >   https://lore.kernel.org/linux-mm/20251013101636.69220-1-21cnbao@gmail.com/
> >
> >   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;
> >       }
> >
> >       /*
>

Thanks
Barry

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@linux.alibaba.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: David Hildenbrand <david@kernel.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Qi Zheng <zhengqi.arch@bytedance.com>
> Cc: Shakeel Butt <shakeel.butt@linux.dev>
> Cc: Lorenzo Stoakes <ljs@kernel.org>
> Cc: Kairui Song <kasong@tencent.com>
> Cc: Axel Rasmussen <axelrasmussen@google.com>
> Cc: Yuanchu Xie <yuanchu@google.com>
> Cc: Wei Xu <weixugc@google.com>
> Co-developed-by: Wang Lian <wanglian@kylinos.cn>
> Co-developed-by: Kunwu Chan <chentao@kylinos.cn>
> Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
> ---
>  -RFC v1 was "mm: net: disable kswapd for high-order network
>   buffer allocation":
>  https://lore.kernel.org/linux-mm/20251013101636.69220-1-21cnbao@gmail.com/
> 
>  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@cmpxchg.org/

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?

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

[Barry Song]

On Wed, Apr 22, 2026 at 11:47 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> 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?

I previously worked for a company that had some data. I’m no
longer engaged with them, but I’m trying to recollect the data
on a new platform I recently moved to. My impression is that
when over-reclamation occurs, the amount of free memory is
actually much higher than what compaction_suitable() would
indicate.

Will present the data once recollection is complete with the
help from Wenchao.

>
> > 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@linux.alibaba.com>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Cc: David Hildenbrand <david@kernel.org>
> > Cc: Michal Hocko <mhocko@kernel.org>
> > Cc: Qi Zheng <zhengqi.arch@bytedance.com>
> > Cc: Shakeel Butt <shakeel.butt@linux.dev>
> > Cc: Lorenzo Stoakes <ljs@kernel.org>
> > Cc: Kairui Song <kasong@tencent.com>
> > Cc: Axel Rasmussen <axelrasmussen@google.com>
> > Cc: Yuanchu Xie <yuanchu@google.com>
> > Cc: Wei Xu <weixugc@google.com>
> > Co-developed-by: Wang Lian <wanglian@kylinos.cn>
> > Co-developed-by: Kunwu Chan <chentao@kylinos.cn>
> > Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
> > ---
> >  -RFC v1 was "mm: net: disable kswapd for high-order network
> >   buffer allocation":
> >  https://lore.kernel.org/linux-mm/20251013101636.69220-1-21cnbao@gmail.com/
> >
> >  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@cmpxchg.org/
>
> 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.

I guess one possibility is to check whether the gap is much
larger than compaction_gap(), to ensure there are sufficient
gaps.

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 4f9668aa8eef..428ec7266aab 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -6969,6 +6969,7 @@ static bool pgdat_balanced(pg_data_t *pgdat, int
order, int highest_zoneidx)
                 * higher-order pages are unavailable, kswapd may still
                 * trigger excessive reclamation.
                 */
+               mark += 2 * num_possible_cpus() * compact_gap(order);
                if (order && compaction_suitable(zone, order, mark,
highest_zoneidx))
                        return true;
        }

On the other hand, “ongoing unsatisfied demand” may not always
make sense. For example, network code may first try higher
orders with the kswapd flag but without the direct reclamation
flag, and then quickly fall back to order-0 allocations if the
higher-order allocation fails. In many cases, higher orders do
not improve network performance, while kswapd performs
unnecessary reclamation and may further trigger more refaults.

>
> 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?

I could investigate this further, but I suspect it is mainly
because we set a very large nr_to_reclaim, assuming we want to
reach the high watermark, while we may already have been above
the high watermark in kswapd_shrink_node()?

        /* Reclaim a number of pages proportional to the number of zones */
        sc->nr_to_reclaim = 0;
        for_each_managed_zone_pgdat(zone, pgdat, z, sc->reclaim_idx) {
                sc->nr_to_reclaim += max(high_wmark_pages(zone),
SWAP_CLUSTER_MAX);
        }

A rough look suggests another place where high-order may affect
scanning is here:

static bool should_abort_scan(struct lruvec *lruvec, struct scan_control *sc)
{
        int i;
        enum zone_watermarks mark;

        if (sc->nr_reclaimed >= max(sc->nr_to_reclaim, compact_gap(sc->order)))
                return true;
       ...
}

So maybe we set something like the following instead, if we are
actually above the high watermark and are only short of
high-order pages?

sc->nr_to_reclaim = min(sc->nr_to_reclaim, 2 * compact_gap(sc->order))

Anyway, let me take a closer look.

Thanks
Barry

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

[Lian Wang]

From: "Lian Wang (ProcessMission)" <lianux.mm@gmail.com>

Hi Johannes, Barry, Baolin, Kairui, everyone,

We revisited the thread and Johannes's earlier analysis on the kswapd/kcompactd
handoff. We agree that a single static `compact_gap()` cannot represent
hundreds or thousands of concurrent higher-order requests. In such workloads,
repeated kswapd wakeups often reflect real unsatisfied demand, and stopping
kswapd too early can shift the burden to direct reclaim and direct compaction.

At the same time, related reclaim issues are still appearing in several
environments:

  - Barry observed order-3 network allocations causing persistent kswapd
    activity and device heating on phones, even though requesters can fall back
    quickly to order-0.
  - Baolin and Kairui have seen similar fragmentation-driven reclaim behavior
    on servers.
  - We have reproduced on PCs and VMs that order-0 memory and total available
    memory can remain plentiful while order-3 blocks are scarce, yet kswapd
    consumes substantial CPU and runs heavy reclaim/scan loops. We have not yet
    reproduced this consistently on our server systems.

There is a useful parallel in Johannes's production work on reclaim storms in
defrag_mode:

https://lore.kernel.org/20260626182215.1107966-1-hannes@cmpxchg.org/

The allocation paths differ, but both threads expose a similar feedback problem:
reclaim may report progress in the number of freed pages, while not making useful
allocation progress for the order, migratetype, or fallback intent that triggered
it.

> The compaction gap is sized for a single allocation, but
> kswapd/kcompactd are a shared resource for potentially hundreds or
> thousands of incoming requests.

We share this concern and do not want to replace the existing incremental
pipeline with one fixed value. We would like to separate two possibilities:

  1. One high-order kswapd wakeup overshoots `compact_gap()` substantially
     before `sc->order` is reset and kcompactd is woken.
  2. Each wakeup makes bounded progress, and the total overhead comes from
     repeated wakeups after best-effort callers have already fallen back to
     order-0.

For the first case, limiting the high-order reclaim scan or per-wakeup reclaim
budget when order-0 watermarks are already satisfied may preserve the repeated-
wakeup pipeline while avoiding a large single-pass overshoot. For the second,
we may need stronger accounting of outstanding higher-order demand, instead of
treating every repeated best-effort attempt as a request that still needs
background progress.

Would it make sense to align on a common, parameterized reproducer and trace set
before choosing between changing `pgdat_balanced()`, bounding the high-order
reclaim budget, or improving demand accounting? A useful matrix could cover:

  - best-effort order-3 allocation with immediate order-0 fallback;
  - sustained and concurrent THP or pageblock-order demand;
  - default mode and defrag_mode;
  - phone, PC/VM, and NUMA server environments.

We can share our reproducer and the PC/VM traces, and adapt the workload to
collect per-wakeup `nr_to_reclaim`, `nr_reclaimed`, `nr_scanned`, order reset,
kcompactd activity, allocation success, and fallback path details. It would be
very helpful if Barry, Baolin, Kairui, and Johannes can tell us which parts of
this matrix and which counters best represent the phone and server cases. Then
we can compare scenarios with aligned measurements and work toward a solution that
does not trade mobile reclaim storms for server direct-reclaim/compaction storms.

Thanks,
Lian

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

[Baolin Wang]

Hi Lian,


On 9/14/26 7:33 PM, Lian Wang wrote:
> From: "Lian Wang (ProcessMission)" <lianux.mm@gmail.com>
> 
> Hi Johannes, Barry, Baolin, Kairui, everyone,

Thanks for raising this issue again.

> We revisited the thread and Johannes's earlier analysis on the kswapd/kcompactd
> handoff. We agree that a single static `compact_gap()` cannot represent
> hundreds or thousands of concurrent higher-order requests. In such workloads,
> repeated kswapd wakeups often reflect real unsatisfied demand, and stopping
> kswapd too early can shift the burden to direct reclaim and direct compaction.
> 
> At the same time, related reclaim issues are still appearing in several
> environments:
> 
>    - Barry observed order-3 network allocations causing persistent kswapd
>      activity and device heating on phones, even though requesters can fall back
>      quickly to order-0.
>    - Baolin and Kairui have seen similar fragmentation-driven reclaim behavior
>      on servers.
>    - We have reproduced on PCs and VMs that order-0 memory and total available
>      memory can remain plentiful while order-3 blocks are scarce, yet kswapd
>      consumes substantial CPU and runs heavy reclaim/scan loops. We have not yet
>      reproduced this consistently on our server systems.
> 
> There is a useful parallel in Johannes's production work on reclaim storms in
> defrag_mode:
> 
> https://lore.kernel.org/20260626182215.1107966-1-hannes@cmpxchg.org/
> 
> The allocation paths differ, but both threads expose a similar feedback problem:
> reclaim may report progress in the number of freed pages, while not making useful
> allocation progress for the order, migratetype, or fallback intent that triggered
> it.
> 
>> The compaction gap is sized for a single allocation, but
>> kswapd/kcompactd are a shared resource for potentially hundreds or
>> thousands of incoming requests.
> 
> We share this concern and do not want to replace the existing incremental
> pipeline with one fixed value. We would like to separate two possibilities:
> 
>    1. One high-order kswapd wakeup overshoots `compact_gap()` substantially
>       before `sc->order` is reset and kcompactd is woken.
>    2. Each wakeup makes bounded progress, and the total overhead comes from
>       repeated wakeups after best-effort callers have already fallen back to
>       order-0.
> 
> For the first case, limiting the high-order reclaim scan or per-wakeup reclaim
> budget when order-0 watermarks are already satisfied may preserve the repeated-
> wakeup pipeline while avoiding a large single-pass overshoot. For the second,
> we may need stronger accounting of outstanding higher-order demand, instead of
> treating every repeated best-effort attempt as a request that still needs
> background progress.
> 
> Would it make sense to align on a common, parameterized reproducer and trace set
> before choosing between changing `pgdat_balanced()`, bounding the high-order
> reclaim budget, or improving demand accounting? A useful matrix could cover:
> 
>    - best-effort order-3 allocation with immediate order-0 fallback;
>    - sustained and concurrent THP or pageblock-order demand;
>    - default mode and defrag_mode;
>    - phone, PC/VM, and NUMA server environments.
> 
> We can share our reproducer and the PC/VM traces, and adapt the workload to
> collect per-wakeup `nr_to_reclaim`, `nr_reclaimed`, `nr_scanned`, order reset,
> kcompactd activity, allocation success, and fallback path details. It would be
> very helpful if Barry, Baolin, Kairui, and Johannes can tell us which parts of
> this matrix and which counters best represent the phone and server cases. Then
> we can compare scenarios with aligned measurements and work toward a solution that
> does not trade mobile reclaim storms for server direct-reclaim/compaction storms.

Like I said before [1], the problem we ran into is that in scenarios 
where fragmentation caused kswapd to be woken up and reclaim too many 
file folios (even when free memory was sufficient), leading to severe 
I/O contention that impacted some applications.

In such fragmentation scenarios, I don't think we should reclaim a large 
number of file folios, instead we should wake up kcompactd to perform 
defragmentation. So in theory, I prefer Barry's earlier proposal to 
adjust 'sc->nr_to_reclaim' [2]. But let's see what Johannes thinks.

Based on our problem, I think we can setup a simple test scenario: one 
workload tries to allocate THP under fragmentation (and free memory was 
sufficient now), while another fio workload accesses file folios 
periodically. In this scenario, kswapd's reclaim should not affect the 
fio workload's performance.

[1] 
https://lore.kernel.org/all/8d4df864-2954-4eb6-b8d7-ae6595646e6e@linux.alibaba.com/
[2] 
https://lore.kernel.org/all/CAGsJ_4x9vkv2G7d-gANf8kqUJVxQh_P9=AJxr98LFUgQeOTqow@mail.gmail.com/

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

[Lian Wang]

From: "Lian Wang (ProcessMission)" <lianux.mm@gmail.com>

Hi Baolin,

Thanks, this gives us a concrete server-side reproducer. We can set up the
fragmented-memory test with one workload requesting THP and another periodically
accessing file folios through fio. We will keep free memory sufficient for
order-0 allocations so that the test isolates fragmentation-driven reclaim,
then measure fio latency and throughput together with kswapd reclaim, kcompactd
activity, direct reclaim/compaction, and THP allocation success.

We previously reproduced the kswapd CPU and reclaim behavior that motivated
Barry's proposal, but we have not yet measured the benefit or possible
regressions of the patch itself. As Johannes pointed out earlier, limiting
high-order reclaim too aggressively can regress sustained or concurrent huge
page requests by moving more work into direct reclaim and compaction. Therefore,
when testing Barry's patch, we would like to cover both the fio interference
case you described and concurrent higher-order allocation pressure.

Let's also wait for Johannes's thoughts, especially on whether bounding
`sc->nr_to_reclaim` should be combined with better accounting of outstanding
higher-order demand. In parallel, we can align the test matrix around:

  - best-effort order-3 network allocations with immediate order-0 fallback
    on phones;
  - order-3 scarcity with sufficient free memory and repeated kswapd wakeups
    on PCs and VMs;
  - fragmented THP allocations competing with file-folio I/O on servers.

We can run the baseline and Barry's patch with the same reclaim, compaction,
allocation-success, CPU, and workload-latency measurements. This should help us
unify the design discussion and avoid optimizing the phone, PC, or server case
at the expense of the others.

Thanks,
Lian

On Tue, 15 Sep 2026 10:30:50 +0800 Baolin Wang <baolin.wang@linux.alibaba.com> wrote:

> In such fragmentation scenarios, I don't think we should reclaim a large
> number of file folios, instead we should wake up kcompactd to perform
> defragmentation. So in theory, I prefer Barry's earlier proposal to
> adjust 'sc->nr_to_reclaim' [2]. But let's see what Johannes thinks.
>
> Based on our problem, I think we can setup a simple test scenario: one
> workload tries to allocate THP under fragmentation (and free memory was
> sufficient now), while another fio workload accesses file folios
> periodically. In this scenario, kswapd's reclaim should not affect the
> fio workload's performance.
>
> [2]
> https://lore.kernel.org/all/CAGsJ_4x9vkv2G7d-gANf8kqUJVxQh_P9=AJxr98LFUgQeOTqow@mail.gmail.com/