Re: [RFC PATCH v2 3/3] mm/zsmalloc: drop class lock before freeing zspage

[Joshua Hahn <joshua.hahnjy@gmail.com>]

On Wed, 27 May 2026 19:59:30 +0800 Wenchao Hao <haowenchao22@gmail.com> wrote:

> From: Xueyuan Chen <xueyuan.chen21@gmail.com>
> 
> Currently in zs_free(), the class->lock is held until the zspage is
> completely freed and the counters are updated. However, freeing pages
> back to the buddy allocator requires acquiring the zone lock.
> 
> Under heavy memory pressure, zone lock contention can be severe. When
> this happens, the CPU holding the class->lock will stall waiting for
> the zone lock, thereby blocking all other CPUs attempting to acquire
> the same class->lock.
> 
> This patch shrinks the critical section of the class->lock to reduce
> lock contention. By moving the actual page freeing process outside the
> class->lock, we can improve the concurrency performance of zs_free().

Hello Wenchao and Xueyuan,

Thank you for the patch. This is a really cool idea and I think it makes
a lot of sense. 

LGTM aside from a few nits, with those resolved please feel free to add
Reviewed-by: Joshua Hahn <joshua.hahnjy@gmail.com>

>  	fullness = fix_fullness_group(class, zspage);
> -	if (fullness == ZS_INUSE_RATIO_0)
> -		free_zspage(pool, class, zspage);
> +	if (fullness == ZS_INUSE_RATIO_0) {
> +		if (trylock_zspage(zspage)) {
> +			remove_zspage(class, zspage);
> +			class_stat_sub(class, ZS_OBJS_ALLOCATED,
> +				class->objs_per_zspage);
> +			zspage_to_free = zspage;
> +		} else
> +			kick_deferred_free(pool);

NIT: We should probably add opening and closing braces here as well since the
if() case is more than one line. 

> +	}
>  
>  	spin_unlock(&class->lock);

Sashiko caught this in [1] and I was confused why the likely() doesn't make
sense here, but I finally realized that zspage_to_free is likely to be set
when comparing trylock_zspage() success vs. failure, but it's unlikely to
enter the ZS_INUSE_RATIO_0 case to begin with, so an unlikely() seems
correct here. 

Thanks again. I hope you have a great day!
Joshua

> +	if (likely(zspage_to_free)) {
> +		__free_zspage_lockless(pool, zspage_to_free);
> +		atomic_long_sub(class->pages_per_zspage, &pool->pages_allocated);
> +	}
>  	cache_free_handle(handle);
>  }
>  EXPORT_SYMBOL_GPL(zs_free);

[1] https://sashiko.dev/#/patchset/20260527115930.3138213-1-haowenchao%40xiaomi.com