linux-kernel - Re: [PATCH v2 12/28] mm: vmstat: use s32 for vm_node_stat_diff in struct per_cpu

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Message-ID: <20200203175818.GF1697@cmpxchg.org>
Date:   Mon, 3 Feb 2020 12:58:18 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Roman Gushchin <guro@...com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        Bharata B Rao <bharata@...ux.ibm.com>,
        Yafang Shao <laoar.shao@...il.com>
Subject: Re: [PATCH v2 12/28] mm: vmstat: use s32 for vm_node_stat_diff in
 struct per_cpu_nodestat

On Mon, Jan 27, 2020 at 09:34:37AM -0800, Roman Gushchin wrote:
> Currently s8 type is used for per-cpu caching of per-node statistics.
> It works fine because the overfill threshold can't exceed 125.
> 
> But if some counters are in bytes (and the next commit in the series
> will convert slab counters to bytes), it's not gonna work:
> value in bytes can easily exceed s8 without exceeding the threshold
> converted to bytes. So to avoid overfilling per-cpu caches and breaking
> vmstats correctness, let's use s32 instead.
> 
> This doesn't affect per-zone statistics. There are no plans to use
> zone-level byte-sized counters, so no reasons to change anything.

Wait, is this still necessary? AFAIU, the node counters will account
full slab pages, including free space, and only the memcg counters
that track actual objects will be in bytes.

Can you please elaborate?