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Subject: [PATCH] mm/compaction: avoid VM_BUG_ON(PageSlab()) in page_mapcount()
From:   Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
To:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>
Date:   Wed, 13 May 2020 17:05:25 +0300
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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);