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From: Andrew Morton <>
Subject: [patch 6/7] mm, hotplug: fix page online with DEBUG_PAGEALLOC compiled but not enabled
Date: Thu, 05 Mar 2020 22:28:42 -0800	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

From: Vlastimil Babka <>
Subject: mm, hotplug: fix page online with DEBUG_PAGEALLOC compiled but not enabled

Commit cd02cf1aceea ("mm/hotplug: fix an imbalance with DEBUG_PAGEALLOC")
fixed memory hotplug with debug_pagealloc enabled, where onlining a page
goes through page freeing, which removes the direct mapping.  Some arches
don't like when the page is not mapped in the first place, so
generic_online_page() maps it first.  This is somewhat wasteful, but
better than special casing page freeing fast paths.

The commit however missed that DEBUG_PAGEALLOC configured doesn't mean
it's actually enabled.  One has to test debug_pagealloc_enabled() since
031bc5743f15 ("mm/debug-pagealloc: make debug-pagealloc boottime
configurable"), or alternatively debug_pagealloc_enabled_static() since
8e57f8acbbd1 ("mm, debug_pagealloc: don't rely on static keys too early"),
but this is not done.

As a result, a s390 kernel with DEBUG_PAGEALLOC configured but not enabled
will crash:

Unable to handle kernel pointer dereference in virtual kernel address space
Failing address: 0000000000000000 TEID: 0000000000000483
Fault in home space mode while using kernel ASCE.
AS:0000001ece13400b R2:000003fff7fd000b R3:000003fff7fcc007 S:000003fff7fd7000 P:000000000000013d
Oops: 0004 ilc:2 [#1] SMP
CPU: 1 PID: 26015 Comm: chmem Kdump: loaded Tainted: GX 5.3.18-5-default #1 SLE15-SP2 (unreleased)
Krnl PSW : 0704e00180000000 0000001ecd281b9e (__kernel_map_pages+0x166/0x188)
R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:2 PM:0 RI:0 EA:3
Krnl GPRS: 0000000000000000 0000000000000800 0000400b00000000 0000000000000100
0000000000000001 0000000000000000 0000000000000002 0000000000000100
0000001ece139230 0000001ecdd98d40 0000400b00000100 0000000000000000
000003ffa17e4000 001fffe0114f7d08 0000001ecd4d93ea 001fffe0114f7b20
Krnl Code: 0000001ecd281b8e: ec17ffff00d8 ahik %r1,%r7,-1
0000001ecd281b94: ec111dbc0355 risbg %r1,%r1,29,188,3
>0000001ecd281b9e: 94fb5006 ni 6(%r5),251
0000001ecd281ba2: 41505008 la %r5,8(%r5)
0000001ecd281ba6: ec51fffc6064 cgrj %r5,%r1,6,1ecd281b9e
0000001ecd281bac: 1a07 ar %r0,%r7
0000001ecd281bae: ec03ff584076 crj %r0,%r3,4,1ecd281a5e
Call Trace:
[<0000001ecd281b9e>] __kernel_map_pages+0x166/0x188
[<0000001ecd4d9516>] online_pages_range+0xf6/0x128
[<0000001ecd2a8186>] walk_system_ram_range+0x7e/0xd8
[<0000001ecda28aae>] online_pages+0x2fe/0x3f0
[<0000001ecd7d02a6>] memory_subsys_online+0x8e/0xc0
[<0000001ecd7add42>] device_online+0x5a/0xc8
[<0000001ecd7d0430>] state_store+0x88/0x118
[<0000001ecd5b9f62>] kernfs_fop_write+0xc2/0x200
[<0000001ecd5064b6>] vfs_write+0x176/0x1e0
[<0000001ecd50676a>] ksys_write+0xa2/0x100
[<0000001ecda315d4>] system_call+0xd8/0x2c8

Fix this by checking debug_pagealloc_enabled_static() before calling
kernel_map_pages(). Backports for kernel before 5.5 should use
debug_pagealloc_enabled() instead. Also add comments.

Fixes: cd02cf1aceea ("mm/hotplug: fix an imbalance with DEBUG_PAGEALLOC")
Signed-off-by: Vlastimil Babka <>
Reported-by: Gerald Schaefer <>
Reviewed-by: David Hildenbrand <>
Cc: Qian Cai <>
Cc: Joonsoo Kim <>
Cc: <>
Signed-off-by: Andrew Morton <>

 include/linux/mm.h  |    4 ++++
 mm/memory_hotplug.c |    8 +++++++-
 2 files changed, 11 insertions(+), 1 deletion(-)

--- a/include/linux/mm.h~mm-hotplug-fix-page-online-with-debug_pagealloc-compiled-but-not-enabled
+++ a/include/linux/mm.h
@@ -2715,6 +2715,10 @@ static inline bool debug_pagealloc_enabl
 extern void __kernel_map_pages(struct page *page, int numpages, int enable);
+ * When called in DEBUG_PAGEALLOC context, the call should most likely be
+ * guarded by debug_pagealloc_enabled() or debug_pagealloc_enabled_static()
+ */
 static inline void
 kernel_map_pages(struct page *page, int numpages, int enable)
--- a/mm/memory_hotplug.c~mm-hotplug-fix-page-online-with-debug_pagealloc-compiled-but-not-enabled
+++ a/mm/memory_hotplug.c
@@ -574,7 +574,13 @@ EXPORT_SYMBOL_GPL(restore_online_page_ca
 void generic_online_page(struct page *page, unsigned int order)
-	kernel_map_pages(page, 1 << order, 1);
+	/*
+	 * Freeing the page with debug_pagealloc enabled will try to unmap it,
+	 * so we should map it first. This is better than introducing a special
+	 * case in page freeing fast path.
+	 */
+	if (debug_pagealloc_enabled_static())
+		kernel_map_pages(page, 1 << order, 1);
 	__free_pages_core(page, order);
 	totalram_pages_add(1UL << order);

  parent reply	other threads:[~2020-03-06  6:28 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-06  6:27 incoming Andrew Morton
2020-03-06  6:28 ` [patch 1/7] mm, numa: fix bad pmd by atomically check for pmd_trans_huge when marking page tables prot_numa Andrew Morton
2020-03-06  6:28 ` [patch 2/7] mm: fix possible PMD dirty bit lost in set_pmd_migration_entry() Andrew Morton
2020-03-06  6:28 ` [patch 3/7] mm: avoid data corruption on CoW fault into PFN-mapped VMA Andrew Morton
2020-03-06  6:28 ` [patch 4/7] fat: fix uninit-memory access for partial initialized inode Andrew Morton
2020-03-06  6:28 ` [patch 5/7] mm/z3fold.c: do not include rwlock.h directly Andrew Morton
2020-03-06  6:28 ` Andrew Morton [this message]
2020-03-06  6:28 ` [patch 7/7] arch/Kconfig: update HAVE_RELIABLE_STACKTRACE description Andrew Morton
2020-03-12  4:12 ` mmotm 2020-03-11-21-11 uploaded Andrew Morton
2020-03-12 15:03   ` mmotm 2020-03-11-21-11 uploaded (sound/soc/codecs/wcd9335.c) Randy Dunlap
2020-03-12 16:59     ` Srinivas Kandagatla
2020-03-13  4:52       ` Masahiro Yamada
2020-03-13  4:49 ` mmotm 2020-03-12-21-49 uploaded Andrew Morton
2020-03-21 22:17 ` mmotm 2020-03-21-15-17 uploaded Andrew Morton

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