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Subject: Re: [RFC PATCH v2 3/5] mm: Add explicit page decrement in exception
 path for isolate_lru_pages
To:     Alexander Duyck <alexander.duyck@gmail.com>
Cc:     yang.shi@linux.alibaba.com, lkp@intel.com, rong.a.chen@intel.com,
        khlebnikov@yandex-team.ru, kirill@shutemov.name, hughd@google.com,
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        hannes@cmpxchg.org, tj@kernel.org, cgroups@vger.kernel.org,
        akpm@linux-foundation.org, richard.weiyang@gmail.com,
        mgorman@techsingularity.net, iamjoonsoo.kim@lge.com
References: <20200819041852.23414.95939.stgit@localhost.localdomain>
 <20200819042722.23414.2654.stgit@localhost.localdomain>
From:   Alex Shi <alex.shi@linux.alibaba.com>
Message-ID: <cc993d93-a5af-dd29-19f4-176ba86000e1@linux.alibaba.com>
Date:   Wed, 19 Aug 2020 15:50:50 +0800
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在 2020/8/19 下午12:27, Alexander Duyck 写道:
> From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> 
> In isolate_lru_pages we have an exception path where if we call
> get_page_unless_zero and that succeeds, but TestClearPageLRU fails we call
> put_page. Normally this would be problematic but due to the way that the
> calls are ordered and the fact that we are holding the LRU lock we know
> that the caller must be holding another reference for the page. Since we
> can assume that we can replace the put_page with a call to
> put_page_testzero contained within a WARN_ON. By doing this we should see
> if we ever leak a page as a result of the reference count somehow hitting
> zero when it shouldn't, and can avoid the overhead and confusion of using
> the full put_page call.
> 
> Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> ---
>  mm/vmscan.c |    9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 5bc0c2322043..3ebe3f9b653b 100644
> --- a/mm/vmscan.c
> +++ 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));

seems WARN_ON is always enabled.

Reviewed-by: Alex Shi <alex.shi@linux.alibaba.com>

>  				goto busy;
>  			}
>  
>