Documentation/vm/hwpoison.txt

   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
 187