Re: [PATCH v6 0/4] mm/zsmalloc: reduce lock contention in zs_free()

[Andrew Morton]

On Fri, 26 Jun 2026 09:49:59 +0800 Wenchao Hao <haowenchao22@xxxxxxxxx> wrote:

> From: Wenchao Hao <haowenchao@xxxxxxxxxx>
> 
> This series reduces lock contention in zs_free(), which dominates the
> unmap path under memory pressure on Android (LMK kills) and on x86
> servers running zswap-heavy workloads.
> 
> The current zs_free() takes pool->lock (rwlock, read side) just to
> look up the size_class for a handle, then takes class->lock and holds
> it across __free_zspage() which can call into the buddy allocator and
> acquire zone->lock.  Two costs follow:
> 
>   * pool->lock reader-counter cacheline bouncing among concurrent
>     zs_free() callers.
>   * class->lock held across folio_put(), so any zone->lock wait
>     fans out to every other zs_free() on the same class.
> 
> The series tackles both:
> 
>   Patch 1: encode size_class index into obj alongside PFN and obj_idx,
>            so zs_free() can locate the class without pool->lock.
>   Patch 2: drop pool->lock from zs_free() on 64-bit; 32-bit unchanged.
>   Patch 3: move zspage page-freeing out of class->lock.
>   Patch 4: document the three free_zspage helper variants that result
>            from the split in patch 3.
> 
> Performance results:
> 
> Test: each process independently mmap 256MB, write data, madvise
> MADV_PAGEOUT to swap out via zram (lzo-rle), then concurrent munmap.
> 
> Raspberry Pi 4B (4-core ARM64 Cortex-A72):
> 
>   mode        Base       Patched     Speedup
>   single      59.0ms     56.0ms      1.05x
>   multi 2p    94.6ms     66.7ms      1.42x
>   multi 4p    202.9ms    110.6ms     1.83x
> 
> x86 (20-core Intel i7-12700, 16 concurrent processes):
> 
>   mode        Base       Patched     Speedup
>   single      11.7ms     9.8ms       1.19x
>   multi 2p    24.1ms     17.2ms      1.40x
>   multi 4p    63.0ms     45.3ms      1.39x

Well that's a nice result.

Sashiko AI review said ....  nothing.  I don't recall seeing that
before ;)

I'll add this series to mm.git for the next step, thanks.