linux-kernel - Re: [PATCH v20 08/20] mm: page_idle_get_page() does not need lru

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Message-ID: <20201102144927.GN27442@casper.infradead.org>
Date:   Mon, 2 Nov 2020 14:49:27 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Alex Shi <alex.shi@...ux.alibaba.com>, akpm@...ux-foundation.org,
        mgorman@...hsingularity.net, tj@...nel.org, hughd@...gle.com,
        khlebnikov@...dex-team.ru, daniel.m.jordan@...cle.com,
        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 v20 08/20] mm: page_idle_get_page() does not need lru_lock

On Mon, Nov 02, 2020 at 09:41:10AM -0500, Johannes Weiner wrote:
> On Thu, Oct 29, 2020 at 06:44:53PM +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(),
> 
> 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?

I don't think it can point to an anon_vma without the ANON bit set.
Minchan's concern in that email was that it might still be NULL.