linux-kernel - Re: [RESEND PATCH v2 01/12] mm: memcontrol: fix NR_ANON

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Date:   Thu, 10 Dec 2020 17:00:45 +0100
From:   Johannes Weiner <hannes@...xchg.org>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     gregkh@...uxfoundation.org, rafael@...nel.org, adobriyan@...il.com,
        akpm@...ux-foundation.org, mhocko@...nel.org,
        vdavydov.dev@...il.com, hughd@...gle.com, will@...nel.org,
        guro@...com, rppt@...nel.org, tglx@...utronix.de, esyr@...hat.com,
        peterx@...hat.com, krisman@...labora.com, surenb@...gle.com,
        avagin@...nvz.org, elver@...gle.com, rdunlap@...radead.org,
        iamjoonsoo.kim@....com, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        cgroups@...r.kernel.org
Subject: Re: [RESEND PATCH v2 01/12] mm: memcontrol: fix NR_ANON_THPS account

On Sun, Dec 06, 2020 at 06:14:40PM +0800, Muchun Song wrote:
> The unit of NR_ANON_THPS is HPAGE_PMD_NR already. So it should inc/dec
> by one rather than nr_pages.

This is a real bug, thanks for catching it.

However, your patch changes the user-visible output of /proc/vmstat!

NR_ANON_THPS isn't just used by memcg, it's a generic accounting item
of the memory subsystem. See this from the Fixes:-patch:

-                       __inc_node_page_state(page, NR_ANON_THPS);
+                       __inc_lruvec_page_state(page, NR_ANON_THPS);

While we've considered /proc/vmstat less official than other files
like meminfo, and have in the past freely added and removed items,
changing the unit of an existing one is not going to work.