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Date:   Mon, 2 Nov 2020 09:41:10 -0500
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Alex Shi <alex.shi@linux.alibaba.com>
Cc:     akpm@linux-foundation.org, mgorman@techsingularity.net,
        tj@kernel.org, hughd@google.com, khlebnikov@yandex-team.ru,
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        kirill@shutemov.name, alexander.duyck@gmail.com,
        rong.a.chen@intel.com, mhocko@suse.com, vdavydov.dev@gmail.com,
        shy828301@gmail.com, Vlastimil Babka <vbabka@suse.cz>,
        Minchan Kim <minchan@kernel.org>
Subject: Re: [PATCH v20 08/20] mm: page_idle_get_page() does not need lru_lock
Message-ID: <20201102144110.GB724984@cmpxchg.org>
References: <1603968305-8026-1-git-send-email-alex.shi@linux.alibaba.com>
 <1603968305-8026-9-git-send-email-alex.shi@linux.alibaba.com>
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On Thu, Oct 29, 2020 at 06:44:53PM +0800, Alex Shi wrote:
> From: Hugh Dickins <hughd@google.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(),

That doesn't seem to be the case in Linus's or Andrew's tree. Am I
missing a dependent patch series?

> 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).

Isn't it possible, as per Minchan's description, for page->mapping to
point to a struct anon_vma without PAGE_MAPPING_ANON set, and rmap
thinking it's looking at a struct address_space?