The patch titled
Subject: mm/compaction: avoid VM_BUG_ON(PageSlab()) in page_mapcount()
has been added to the -mm tree. Its filename is
mm-compaction-avoid-vm_bug_onpageslab-in-page_mapcount.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-compaction-avoid-vm_bug_onpageslab-in-page_mapcount.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-compaction-avoid-vm_bug_onpageslab-in-page_mapcount.patch
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------------------------------------------------------
From: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Subject: mm/compaction: avoid VM_BUG_ON(PageSlab()) in page_mapcount()
isolate_migratepages_block() runs some checks out of lru_lock when
choosing 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.
Link: http://lkml.kernel.org/r/158937872515.474360.5066096871639561424.stgit@buzz
Fixes: 119d6d59dcc0 ("mm, compaction: avoid isolating pinned pages")
Fixes: 1d148e218a0d ("mm: add VM_BUG_ON_PAGE() to page_mapcount()")
Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/compaction.c | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)
--- a/mm/compaction.c~mm-compaction-avoid-vm_bug_onpageslab-in-page_mapcount
+++ a/mm/compaction.c
@@ -935,12 +935,16 @@ isolate_migratepages_block(struct compac
}
/*
- * 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 compac
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);
_
Patches currently in -mm which might be from khlebnikov@xxxxxxxxxxxxxx are
mm-compaction-avoid-vm_bug_onpageslab-in-page_mapcount.patch
kernel-watchdog-flush-all-printk-nmi-buffers-when-hardlockup-detected.patch
doc-cgroup-update-note-about-conditions-when-oom-killer-is-invoked.patch
