Re: [PATCH v4 1/2] memcg: acquire peaks_lock when reading memory.peak

[Muchun Song <muchun.song@linux.dev>]

> On Aug 14, 2026, at 11:30, Ridong Chen <ridong.chen@linux.dev> wrote:
> 
> From: Ridong Chen <chenridong@xiaomi.com>
> 
> Sashiko reported that a reader can transiently observe a lower peak
> within a race window [1]. peak_show() returns
> max(local_watermark, ofp->value), but peak_write() updates those two
> under peaks_lock while the reader takes no lock. The interleaving is:
> 
>  writer (reset on fd A)                 reader (fd B)
>  ----------------------                 -------------
>  usage = page_counter_read(pc)
>  WRITE_ONCE(local_watermark, usage)
>  // watermark lowered to usage
>                                         lw  = READ_ONCE(local_watermark)
>                                         // sees the lowered usage
>                                         val = READ_ONCE(ofp->value)
>                                         // B's value not updated yet
>                                         return max(lw, val)
>                                         // both low -> low peak
>  WRITE_ONCE(peer_ctx->value, usage)
>  // B updated, but too late
> 
> Fix it by acquiring peaks_lock when reading the peak, so the reader sees
> a consistent snapshot of local_watermark and the per-fd values. The same
> race applies to memory.swap.peak, which shares peaks_lock and the
> peak_write() path, so take the lock there as well.
> 
> [1] https://sashiko.dev/#/patchset/20260730115314.1069089-1-ridong.chen@linux.dev?part=1
> Fixes: c6f53ed8f213 ("mm, memcg: cg2 memory{.swap,}.peak write handlers")
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> Acked-by: Shakeel Butt <shakeel.butt@linux.dev>

Reviewed-by: Muchun Song <muchun.song@linux.dev>

Thanks