linux-kernel - Re: [PATCH v21 07/19] mm: page_idle_get_page() does not need lru

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Date:   Tue, 10 Nov 2020 14:01:36 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Alex Shi <alex.shi@...ux.alibaba.com>
Cc:     akpm@...ux-foundation.org, mgorman@...hsingularity.net,
        tj@...nel.org, hughd@...gle.com, khlebnikov@...dex-team.ru,
        daniel.m.jordan@...cle.com, willy@...radead.org, lkp@...el.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, shakeelb@...gle.com,
        iamjoonsoo.kim@....com, richard.weiyang@...il.com,
        kirill@...temov.name, alexander.duyck@...il.com,
        rong.a.chen@...el.com, mhocko@...e.com, vdavydov.dev@...il.com,
        shy828301@...il.com, Vlastimil Babka <vbabka@...e.cz>,
        Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH v21 07/19] mm: page_idle_get_page() does not need lru_lock

On Thu, Nov 05, 2020 at 04:55:37PM +0800, Alex Shi wrote:
> From: Hugh Dickins <hughd@...gle.com>
> 
> It is necessary for page_idle_get_page() to recheck PageLRU() after
> get_page_unless_zero(), but holding lru_lock around that serves no
> useful purpose, and adds to lru_lock contention: delete it.
> 
> See https://lore.kernel.org/lkml/20150504031722.GA2768@blaptop for the
> discussion that led to lru_lock there; but __page_set_anon_rmap() now
> uses WRITE_ONCE(), and I see no other risk in page_idle_clear_pte_refs()
> using rmap_walk() (beyond the risk of racing PageAnon->PageKsm, mostly
> but not entirely prevented by page_count() check in ksm.c's
> write_protect_page(): that risk being shared with page_referenced() and
> not helped by lru_lock).
> 
> Signed-off-by: Hugh Dickins <hughd@...gle.com>
> Signed-off-by: Alex Shi <alex.shi@...ux.alibaba.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Vladimir Davydov <vdavydov.dev@...il.com>
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Cc: Minchan Kim <minchan@...nel.org>
> Cc: Alex Shi <alex.shi@...ux.alibaba.com>
> Cc: linux-mm@...ck.org
> Cc: linux-kernel@...r.kernel.org

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