1.. hwpoison:
2
3========
4hwpoison
5========
6
7What is hwpoison?
8=================
9
10Upcoming Intel CPUs have support for recovering from some memory errors
11(``MCA recovery``). This requires the OS to declare a page "poisoned",
12kill the processes associated with it and avoid using it in the future.
13
14This patchkit implements the necessary infrastructure in the VM.
15
16To quote the overview comment::
17
18	High level machine check handler. Handles pages reported by the
19	hardware as being corrupted usually due to a 2bit ECC memory or cache
20	failure.
21
22	This focusses on pages detected as corrupted in the background.
23	When the current CPU tries to consume corruption the currently
24	running process can just be killed directly instead. This implies
25	that if the error cannot be handled for some reason it's safe to
26	just ignore it because no corruption has been consumed yet. Instead
27	when that happens another machine check will happen.
28
29	Handles page cache pages in various states. The tricky part
30	here is that we can access any page asynchronous to other VM
31	users, because memory failures could happen anytime and anywhere,
32	possibly violating some of their assumptions. This is why this code
33	has to be extremely careful. Generally it tries to use normal locking
34	rules, as in get the standard locks, even if that means the
35	error handling takes potentially a long time.
36
37	Some of the operations here are somewhat inefficient and have non
38	linear algorithmic complexity, because the data structures have not
39	been optimized for this case. This is in particular the case
40	for the mapping from a vma to a process. Since this case is expected
41	to be rare we hope we can get away with this.
42
43The code consists of a the high level handler in mm/memory-failure.c,
44a new page poison bit and various checks in the VM to handle poisoned
45pages.
46
47The main target right now is KVM guests, but it works for all kinds
48of applications. KVM support requires a recent qemu-kvm release.
49
50For the KVM use there was need for a new signal type so that
51KVM can inject the machine check into the guest with the proper
52address. This in theory allows other applications to handle
53memory failures too. The expection is that near all applications
54won't do that, but some very specialized ones might.
55
56Failure recovery modes
57======================
58
59There are two (actually three) modes memory failure recovery can be in:
60
61vm.memory_failure_recovery sysctl set to zero:
62	All memory failures cause a panic. Do not attempt recovery.
63
64early kill
65	(can be controlled globally and per process)
66	Send SIGBUS to the application as soon as the error is detected
67	This allows applications who can process memory errors in a gentle
68	way (e.g. drop affected object)
69	This is the mode used by KVM qemu.
70
71late kill
72	Send SIGBUS when the application runs into the corrupted page.
73	This is best for memory error unaware applications and default
74	Note some pages are always handled as late kill.
75
76User control
77============
78
79vm.memory_failure_recovery
80	See sysctl.txt
81
82vm.memory_failure_early_kill
83	Enable early kill mode globally
84
85PR_MCE_KILL
86	Set early/late kill mode/revert to system default
87
88	arg1: PR_MCE_KILL_CLEAR:
89		Revert to system default
90	arg1: PR_MCE_KILL_SET:
91		arg2 defines thread specific mode
92
93		PR_MCE_KILL_EARLY:
94			Early kill
95		PR_MCE_KILL_LATE:
96			Late kill
97		PR_MCE_KILL_DEFAULT
98			Use system global default
99
100	Note that if you want to have a dedicated thread which handles
101	the SIGBUS(BUS_MCEERR_AO) on behalf of the process, you should
102	call prctl(PR_MCE_KILL_EARLY) on the designated thread. Otherwise,
103	the SIGBUS is sent to the main thread.
104
105PR_MCE_KILL_GET
106	return current mode
107
108Testing
109=======
110
111* madvise(MADV_HWPOISON, ....) (as root) - Poison a page in the
112  process for testing
113
114* hwpoison-inject module through debugfs ``/sys/kernel/debug/hwpoison/``
115
116  corrupt-pfn
117	Inject hwpoison fault at PFN echoed into this file. This does
118	some early filtering to avoid corrupted unintended pages in test suites.
119
120  unpoison-pfn
121	Software-unpoison page at PFN echoed into this file. This way
122	a page can be reused again.  This only works for Linux
123	injected failures, not for real memory failures. Once any hardware
124	memory failure happens, this feature is disabled.
125
126  Note these injection interfaces are not stable and might change between
127  kernel versions
128
129  corrupt-filter-dev-major, corrupt-filter-dev-minor
130	Only handle memory failures to pages associated with the file
131	system defined by block device major/minor.  -1U is the
132	wildcard value.  This should be only used for testing with
133	artificial injection.
134
135  corrupt-filter-memcg
136	Limit injection to pages owned by memgroup. Specified by inode
137	number of the memcg.
138
139	Example::
140
141		mkdir /sys/fs/cgroup/mem/hwpoison
142
143	        usemem -m 100 -s 1000 &
144		echo `jobs -p` > /sys/fs/cgroup/mem/hwpoison/tasks
145
146		memcg_ino=$(ls -id /sys/fs/cgroup/mem/hwpoison | cut -f1 -d' ')
147		echo $memcg_ino > /debug/hwpoison/corrupt-filter-memcg
148
149		page-types -p `pidof init`   --hwpoison  # shall do nothing
150		page-types -p `pidof usemem` --hwpoison  # poison its pages
151
152  corrupt-filter-flags-mask, corrupt-filter-flags-value
153	When specified, only poison pages if ((page_flags & mask) ==
154	value).  This allows stress testing of many kinds of
155	pages. The page_flags are the same as in /proc/kpageflags. The
156	flag bits are defined in include/linux/kernel-page-flags.h and
157	documented in Documentation/admin-guide/mm/pagemap.rst
158
159* Architecture specific MCE injector
160
161  x86 has mce-inject, mce-test
162
163  Some portable hwpoison test programs in mce-test, see below.
164
165References
166==========
167
168http://halobates.de/mce-lc09-2.pdf
169	Overview presentation from LinuxCon 09
170
171git://git.kernel.org/pub/scm/utils/cpu/mce/mce-test.git
172	Test suite (hwpoison specific portable tests in tsrc)
173
174git://git.kernel.org/pub/scm/utils/cpu/mce/mce-inject.git
175	x86 specific injector
176
177
178Limitations
179===========
180- Not all page types are supported and never will. Most kernel internal
181  objects cannot be recovered, only LRU pages for now.
182
183---
184Andi Kleen, Oct 2009
185