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Date:   Thu, 5 Nov 2020 10:36:49 -0500
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Alex Shi <alex.shi@linux.alibaba.com>
Cc:     Matthew Wilcox <willy@infradead.org>, akpm@linux-foundation.org,
        mgorman@techsingularity.net, tj@kernel.org, hughd@google.com,
        khlebnikov@yandex-team.ru, daniel.m.jordan@oracle.com,
        lkp@intel.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        cgroups@vger.kernel.org, shakeelb@google.com,
        iamjoonsoo.kim@lge.com, richard.weiyang@gmail.com,
        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: <20201105153649.GC744831@cmpxchg.org>
References: <1603968305-8026-1-git-send-email-alex.shi@linux.alibaba.com>
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 <20201102144927.GN27442@casper.infradead.org>
 <20201102202003.GA740958@cmpxchg.org>
 <b4038b87-cf5a-fcb7-06f4-b98874029615@linux.alibaba.com>
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On Thu, Nov 05, 2020 at 01:03:18PM +0800, Alex Shi wrote:
> 
> 
> 在 2020/11/5 下午12:57, Matthew Wilcox 写道:
> > On Thu, Nov 05, 2020 at 12:52:05PM +0800, Alex Shi wrote:
> >> @@ -1054,8 +1054,27 @@ static void __page_set_anon_rmap(struct page *page,
> >>  	if (!exclusive)
> >>  		anon_vma = anon_vma->root;
> >>  
> >> +	/*
> >> +	 * w/o the WRITE_ONCE here the following scenario may happens due to
> >> +	 * store reordering.
> >> +	 *
> >> +	 *      CPU 0                                          CPU 1
> >> +	 *
> >> +	 * do_anonymous_page				page_idle_clear_pte_refs
> >> +	 *   __page_set_anon_rmap
> >> +	 *     page->mapping = anon_vma + PAGE_MAPPING_ANON
> >> +	 *   lru_cache_add_inactive_or_unevictable()
> >> +	 *     SetPageLRU(page)
> >> +	 *                                               rmap_walk
> >> +	 *                                                if PageAnon(page)
> >> +	 *
> >> +	 *  The 'SetPageLRU' may reordered before page->mapping setting, and
> >> +	 *  page->mapping may set with anon_vma, w/o anon bit, then rmap_walk
> >> +	 *  may goes to rmap_walk_file() for a anon page.
> >> +	 */
> >> +
> >>  	anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
> >> -	page->mapping = (struct address_space *) anon_vma;
> >> +	WRITE_ONCE(page->mapping, (struct address_space *) anon_vma);
> >>  	page->index = linear_page_index(vma, address);
> >>  }
> > 
> > I don't like these verbose comments with detailed descriptions in
> > the source code.  They're fine in changelogs, but they clutter the
> > code, and they get outdated really quickly.  My preference is for
> > something more brief:
> > 
> > 	/*
> > 	 * Prevent page->mapping from pointing to an anon_vma without
> > 	 * the PAGE_MAPPING_ANON bit set.  This could happen if the
> > 	 * compiler stores anon_vma and then adds PAGE_MAPPING_ANON to it.
> > 	 */
> > 

Yeah, I don't think this scenario warrants the full race diagram in
the code itself.

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.
	 */