Messages in this thread | | | Subject | Re: [PATCH] mm/compaction: avoid VM_BUG_ON(PageSlab()) in page_mapcount() | From | Konstantin Khlebnikov <> | 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);
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