linux-kernel - Re: [PATCH 03/18] mm: memcontrol: drop @compound parameter from memcg charging API

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Message-ID: <CALvZod6433GCgVwV7=h5zoByKj7ANoox9FyB6m1_k63KJJnyMQ@mail.gmail.com>
Date:   Wed, 22 Apr 2020 10:30:59 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Joonsoo Kim <js1304@...il.com>,
        Alex Shi <alex.shi@...ux.alibaba.com>,
        Hugh Dickins <hughd@...gle.com>,
        Michal Hocko <mhocko@...e.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Roman Gushchin <guro@...com>, Linux MM <linux-mm@...ck.org>,
        Cgroups <cgroups@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH 03/18] mm: memcontrol: drop @compound parameter from memcg
 charging API

On Mon, Apr 20, 2020 at 3:11 PM Johannes Weiner <hannes@...xchg.org> wrote:
>
> The memcg charging API carries a boolean @compound parameter that
> tells whether the page we're dealing with is a hugepage.
> mem_cgroup_commit_charge() has another boolean @lrucare that indicates
> whether the page needs LRU locking or not while charging. The majority
> of callsites know those parameters at compile time, which results in a
> lot of naked "false, false" argument lists. This makes for cryptic
> code and is a breeding ground for subtle mistakes.
>
> Thankfully, the huge page state can be inferred from the page itself
> and doesn't need to be passed along. This is safe because charging
> completes before the page is published and somebody may split it.
>
> Simplify the callsites by removing @compound, and let memcg infer the
> state by using hpage_nr_pages() unconditionally. That function does
> PageTransHuge() to identify huge pages, which also helpfully asserts
> that nobody passes in tail pages by accident.
>
> The following patches will introduce a new charging API, best not to
> carry over unnecessary weight.
>
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>

Reviewed-by: Shakeel Butt <shakeelb@...gle.com>