From: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
To: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
Andrew Morton <akpm@linux-foundation.org>
Cc: Hugh Dickins <hughd@google.com>, Vlastimil Babka <vbabka@suse.cz>,
David Rientjes <rientjes@google.com>,
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Subject: [PATCH v2] mm: remove VM_BUG_ON(PageSlab()) from page_mapcount()
Date: Sun, 24 May 2020 16:43:18 +0300 [thread overview]
Message-ID: <159032779896.957378.7852761411265662220.stgit@buzz> (raw)
Replace superfluous VM_BUG_ON() with comment about correct usage.
Technically reverts commit 1d148e218a0d0566b1c06f2f45f1436d53b049b2
("mm: add VM_BUG_ON_PAGE() to page_mapcount()"), but context have changed.
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
Code in isolate_migratepages_block() was added in commit 119d6d59dcc0
("mm, compaction: avoid isolating pinned pages") before adding VM_BUG_ON
into page_mapcount().
This race has been predicted in 2015 by Vlastimil Babka (see link below).
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Fixes: 1d148e218a0d ("mm: add VM_BUG_ON_PAGE() to page_mapcount()")
Link: https://lore.kernel.org/lkml/557710E1.6060103@suse.cz/
Link: https://lore.kernel.org/linux-mm/158937872515.474360.5066096871639561424.stgit@buzz/T/ (v1)
---
include/linux/mm.h | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 5a323422d783..95f777f482ac 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -782,6 +782,11 @@ static inline void *kvcalloc(size_t n, size_t size, gfp_t flags)
extern void kvfree(const void *addr);
+/*
+ * Mapcount of compound page as a whole, not includes mapped sub-pages.
+ *
+ * Must be called only for compound pages or any their tail sub-pages.
+ */
static inline int compound_mapcount(struct page *page)
{
VM_BUG_ON_PAGE(!PageCompound(page), page);
@@ -801,10 +806,15 @@ static inline void page_mapcount_reset(struct page *page)
int __page_mapcount(struct page *page);
+/*
+ * Mapcount of 0-order page, for sub-page includes compound_mapcount().
+ *
+ * Result is undefined for pages which cannot be mapped into userspace.
+ * For example SLAB or special types of pages. See function page_has_type().
+ * They use this place in struct page differently.
+ */
static inline int page_mapcount(struct page *page)
{
- VM_BUG_ON_PAGE(PageSlab(page), page);
-
if (unlikely(PageCompound(page)))
return __page_mapcount(page);
return atomic_read(&page->_mapcount) + 1;
next reply other threads:[~2020-05-24 13:43 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-24 13:43 Konstantin Khlebnikov [this message]
2020-05-24 18:49 ` [PATCH v2] mm: remove VM_BUG_ON(PageSlab()) from page_mapcount() Hugh Dickins
2020-05-25 4:44 ` Kirill A. Shutemov
2020-05-25 7:44 ` Vlastimil Babka
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