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

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Fixes: 119d6d59dcc0 ("mm, compaction: avoid isolating pinned pages")
Fixes: 1d148e218a0d ("mm: add VM_BUG_ON_PAGE() to page_mapcount()")
---
mm/compaction.c | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 46f0fcc93081..91bb87fd9420 100644
--- 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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