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

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Date:   Thu, 5 Nov 2020 15:43:53 +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 Thu, Nov 05, 2020 at 10:36:49AM -0500, Johannes Weiner wrote:
> But the code is highly specific - synchronizing one struct page member
> for one particular use case. Let's keep at least a reference to what
> we are synchronizing against. There is a non-zero chance that if the
> comment goes out of date, so does the code. How about this?
> 
> 	/*
> 	 * page_idle does a lockless/optimistic rmap scan on page->mapping.
> 	 * Make sure the compiler doesn't split the stores of anon_vma and
> 	 * the PAGE_MAPPING_ANON type identifier, otherwise the rmap code
> 	 * could mistake the mapping for a struct address_space and crash.
> 	 */

Fine by me!  There may be other cases where seeing a split store would
be bad, so I didn't want to call out page_idle explicitly.  But if you
want to, I'm happy with this comment.