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Subject: Re: [PATCH 1/5] mm/vmscan: put the redirtied MADV_FREE pages back to
 anonymous LRU list
To:     Michal Hocko <mhocko@suse.com>
CC:     <akpm@linux-foundation.org>, <hannes@cmpxchg.org>,
        <vbabka@suse.cz>, <axboe@kernel.dk>, <iamjoonsoo.kim@lge.com>,
        <alexs@kernel.org>, <apopple@nvidia.com>, <willy@infradead.org>,
        <minchan@kernel.org>, <david@redhat.com>, <shli@fb.com>,
        <hillf.zj@alibaba-inc.com>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>
References: <20210710100329.49174-1-linmiaohe@huawei.com>
 <20210710100329.49174-2-linmiaohe@huawei.com>
 <YOvtmy9ggJA4KUIQ@dhcp22.suse.cz>
From:   Miaohe Lin <linmiaohe@huawei.com>
Message-ID: <9409189e-44f7-2608-68af-851629b6d453@huawei.com>
Date:   Mon, 12 Jul 2021 19:03:39 +0800
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On 2021/7/12 15:22, Michal Hocko wrote:
> On Sat 10-07-21 18:03:25, Miaohe Lin wrote:
>> If the MADV_FREE pages are redirtied before they could be reclaimed, put
>> the pages back to anonymous LRU list by setting SwapBacked flag and the
>> pages will be reclaimed in normal swapout way. Otherwise MADV_FREE pages
>> won't be reclaimed as expected.
> 
> Could you describe problem which you are trying to address? What does it
> mean that pages won't be reclaimed as expected?
> 

In fact, this is not a bug and harmless. But it looks buggy as it didn't perform
the expected ops from code view. Lazyfree (MADV_FREE) pages are clean anonymous
pages. They have SwapBacked flag cleared to distinguish normal anonymous pages.
When the MADV_FREE pages are redirtied before they could be reclaimed, the pages
should be put back to anonymous LRU list by setting SwapBacked flag, thus the
pages will be reclaimed in normal swapout way.

Many thanks for review and reply.

> Also why is SetPageSwapBacked in shrink_page_list insufficient?
> 
>> Fixes: 802a3a92ad7a ("mm: reclaim MADV_FREE pages")
>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>> ---
>>  mm/vmscan.c | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index a7602f71ec04..6483fe0e2065 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -1628,6 +1628,7 @@ static unsigned int shrink_page_list(struct list_head *page_list,
>>  			if (!page_ref_freeze(page, 1))
>>  				goto keep_locked;
>>  			if (PageDirty(page)) {
>> +				SetPageSwapBacked(page);
>>  				page_ref_unfreeze(page, 1);
>>  				goto keep_locked;
>>  			}
>> -- 
>> 2.23.0
>