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

[Konstantin Khlebnikov]

On 13/05/2020 21.32, Andrew Morton wrote:

On Wed, 13 May 2020 17:05:25 +0300 Konstantin Khlebnikov 
<khlebni...@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?

Yep, but probably I'm missing something.

It seems bug is there for a long time and nobody seen it.
Am I the only one using COONFIG_DEBUG_VM=y everywhere? =)

--- 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);