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

> --- 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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