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Documentation / vm / hwpoison.txt




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Based on kernel version 3.16. Page generated on 2014-08-06 21:41 EST.

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