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