Re: [RFC PATCH v2 0/4] mm/zsmalloc: reduce zs_free() latency on swap release path

[Xueyuan Chen <xueyuan.chen21@gmail.com>]

On Tue, Apr 21, 2026 at 11:25:17AM -0700, Nhat Pham wrote:

[...]

>Hmm, free_zspage() and kmem_cache_free().
>
>* kmem_cache_free() is just handle freeing. Bulk-freeing?
>
>* free_zspage() looks like just ordinary teardown work :( Seems like
>we're not spinning any lock here - we just try lock the backing pages,
>and the rest is normal work. Not sure how to optimize this - perhaps
>deferring is the only way.
>
>

Hi Nhat,

Currently, free_zspage() is called while holding the class->lock. 
However, free_zspage() eventually invokes folio_put(), which may acquire
the zone->lock.

This creates a nested lock dependency. If multiple CPUs contend for the
same class->lock and the current holder is stalled waiting for the
zone->lock, it significantly extends the hold time of the class->lock.
This causes other CPUs to wait much longer.

Here is the ftrace data showing the severe contention on class->lock.
Under contention, the time spent in queued_spin_lock_slowpath() jumps 
from ~1.3us to over 30us, significantly increasing the total latency
of zs_free().

  7)               |  zs_free() {
  7)   0.220 us    |    _raw_read_lock();
  7)               |    _raw_spin_lock() {
  7)   1.320 us    |      queued_spin_lock_slowpath();
  7)   1.820 us    |    }
  7)   0.170 us    |    _raw_read_unlock();
  7)   0.170 us    |    obj_free();
  7)   0.190 us    |    fix_fullness_group();
  7)   0.150 us    |    _raw_spin_unlock();
  7)   0.170 us    |    kmem_cache_free();
  7)   4.610 us    |  }

---------------------------------------------------------

  7)               |  zs_free() {
  7)   0.230 us    |    _raw_read_lock();
  7)               |    _raw_spin_lock() {
  7) + 30.100 us   |      queued_spin_lock_slowpath();
  7) + 30.600 us   |    }
  7)   0.200 us    |    _raw_read_unlock();
  7)   0.170 us    |    obj_free();
  7)   0.170 us    |    fix_fullness_group();
  7)   0.170 us    |    _raw_spin_unlock();
  7)   0.210 us    |    kmem_cache_free();
  7) + 33.850 us   |  }

Best regards,
Xueyuan