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Date:   Wed, 9 Sep 2020 02:01:18 +0100
From:   Matthew Wilcox <willy@infradead.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,
        daniel.m.jordan@oracle.com, hannes@cmpxchg.org, 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,
        Alexander Duyck <alexander.h.duyck@linux.intel.com>
Subject: Re: [PATCH v18 31/32] mm: Add explicit page decrement in exception
 path for isolate_lru_pages
Message-ID: <20200909010118.GB6583@casper.infradead.org>
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 <1598273705-69124-32-git-send-email-alex.shi@linux.alibaba.com>
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On Mon, Aug 24, 2020 at 08:55:04PM +0800, Alex Shi wrote:
> +++ b/mm/vmscan.c
> @@ -1688,10 +1688,13 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>  
>  			if (!TestClearPageLRU(page)) {
>  				/*
> -				 * This page may in other isolation path,
> -				 * but we still hold lru_lock.
> +				 * This page is being isolated in another
> +				 * thread, but we still hold lru_lock. The
> +				 * other thread must be holding a reference
> +				 * to the page so this should never hit a
> +				 * reference count of 0.
>  				 */
> -				put_page(page);
> +				WARN_ON(put_page_testzero(page));
>  				goto busy;

I read Hugh's review and that led me to take a look at this.  We don't
do it like this.  Use the same pattern as elsewhere in mm:

        page_ref_sub(page, nr);
        VM_BUG_ON_PAGE(page_count(page) <= 0, page);