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Subject: Re: [PATCH] mm/compaction: avoid VM_BUG_ON(PageSlab()) in
 page_mapcount()
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Minchan Kim <minchan@kernel.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        David Rientjes <rientjes@google.com>,
        Mel Gorman <mgorman@techsingularity.net>
References: <158937872515.474360.5066096871639561424.stgit@buzz>
 <20200513113256.44ac9ce4c51a7ec1a5c5ba40@linux-foundation.org>
From:   Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Message-ID: <9420f90c-e423-475c-4b79-8287b76149d9@yandex-team.ru>
Date:   Wed, 13 May 2020 22:28:00 +0300
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On 13/05/2020 21.32, Andrew Morton wrote:
> On Wed, 13 May 2020 17:05:25 +0300 Konstantin Khlebnikov <khlebnikov@yandex-team.ru> wrote:
> 
>> Function isolate_migratepages_block() runs some checks out of lru_lock
>> when choose pages for migration. After checking PageLRU() it checks extra
>> page references by comparing page_count() and page_mapcount(). Between
>> these two checks page could be removed from lru, freed and taken by slab.
>>
>> As a result this race triggers VM_BUG_ON(PageSlab()) in page_mapcount().
>> Race window is tiny. For certain workload this happens around once a year.
>>
>>
>>   page:ffffea0105ca9380 count:1 mapcount:0 mapping:ffff88ff7712c180 index:0x0 compound_mapcount: 0
>>   flags: 0x500000000008100(slab|head)
>>   raw: 0500000000008100 dead000000000100 dead000000000200 ffff88ff7712c180
>>   raw: 0000000000000000 0000000080200020 00000001ffffffff 0000000000000000
>>   page dumped because: VM_BUG_ON_PAGE(PageSlab(page))
>>   ------------[ cut here ]------------
>>   kernel BUG at ./include/linux/mm.h:628!
>>   invalid opcode: 0000 [#1] SMP NOPTI
>>   CPU: 77 PID: 504 Comm: kcompactd1 Tainted: G        W         4.19.109-27 #1
>>   Hardware name: Yandex T175-N41-Y3N/MY81-EX0-Y3N, BIOS R05 06/20/2019
>>   RIP: 0010:isolate_migratepages_block+0x986/0x9b0
>>
>>
>> To fix just opencode page_mapcount() in racy check for 0-order case and
>> recheck carefully under lru_lock when page cannot escape from lru.
>>
>> Also add checking extra references for file pages and swap cache.
> 
> It sounds like a cc:stable is appropriate?

Yep, but probably I'm missing something.

It seems bug is there for a long time and nobody seen it.
Am I the only one using COONFIG_DEBUG_VM=y everywhere? =)

> 
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -935,12 +935,16 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>>   		}
>>   
>>   		/*
>> -		 * Migration will fail if an anonymous page is pinned in memory,
>> +		 * Migration will fail if an page is pinned in memory,
>>   		 * so avoid taking lru_lock and isolating it unnecessarily in an
>> -		 * admittedly racy check.
>> +		 * admittedly racy check simplest case for 0-order pages.
>> +		 *
>> +		 * Open code page_mapcount() to avoid VM_BUG_ON(PageSlab(page)).
>> +		 * Page could have extra reference from mapping or swap cache.
>>   		 */
>> -		if (!page_mapping(page) &&
>> -		    page_count(page) > page_mapcount(page))
>> +		if (!PageCompound(page) &&
>> +		    page_count(page) > atomic_read(&page->_mapcount) + 1 +
>> +				(!PageAnon(page) || PageSwapCache(page)))
>>   			goto isolate_fail;
>>   
>>   		/*
>> @@ -975,6 +979,11 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>>   				low_pfn += compound_nr(page) - 1;
>>   				goto isolate_fail;
>>   			}
>> +
>> +			/* Recheck page extra references under lock */
>> +			if (page_count(page) > page_mapcount(page) +
>> +				    (!PageAnon(page) || PageSwapCache(page)))
>> +				goto isolate_fail;
>>   		}
>>   
>>   		lruvec = mem_cgroup_page_lruvec(page, pgdat);