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? > --- 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);
