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Date:   Fri, 2 Apr 2021 11:22:11 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Muchun Song <songmuchun@bytedance.com>
Cc:     guro@fb.com, mhocko@kernel.org, akpm@linux-foundation.org,
        shakeelb@google.com, vdavydov.dev@gmail.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        duanxiongchun@bytedance.com
Subject: Re: [RFC PATCH 03/15] mm: memcontrol: remove the pgdata parameter of
 mem_cgroup_page_lruvec
Message-ID: <YGc2ozMAYph04nm2@cmpxchg.org>
References: <20210330101531.82752-1-songmuchun@bytedance.com>
 <20210330101531.82752-4-songmuchun@bytedance.com>
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On Tue, Mar 30, 2021 at 06:15:19PM +0800, Muchun Song wrote:
> All the callers of mem_cgroup_page_lruvec() just pass page_pgdat(page)
> as the 2nd parameter to it (except isolate_migratepages_block()). But
> for isolate_migratepages_block(), the page_pgdat(page) is also equal
> to the local variable of @pgdat. So mem_cgroup_page_lruvec() do not
> need the pgdat parameter. Just remove it to simplify the code.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>

There is a minor benefit in not doing the page_pgdat() lookup twice.
But I think it only really matters on 32-bit NUMA, where we have more
than one node but not enough space in page->flags to store the nid,
and so page_to_nid() is out-of-line and needs to go through section.

Those setups aren't really around anymore, and certainly don't justify
the hassle (and potential bugs) of the awkward interface.

Acked-by: Johannes Weiner <hannes@cmpxchg.org>