lkml.org 
[lkml]   [2020]   [May]   [13]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH] mm/compaction: avoid VM_BUG_ON(PageSlab()) in page_mapcount()
From
Date
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);

\
 
 \ /
  Last update: 2020-05-13 21:28    [W:0.121 / U:2.608 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site