Re: [PATCH] mm/compaction: avoid VM_BUG_ON(PageSlab()) in page_mapcount()

[Vlastimil Babka]

On 5/13/20 9:28 PM, Konstantin Khlebnikov wrote:
> On 13/05/2020 21.32, Andrew Morton wrote:
>> On Wed, 13 May 2020 17:05:25 +0300 Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx> 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? =)

Unless things changed, Fedora kernels had it enabled, AFAIK