Documentation/vm/hwpoison.rst

   1 .. hwpoison:
   2
   3 ========
   4 hwpoison
   5 ========
   6
   7 What is hwpoison?
   8 =================
   9
  10 Upcoming Intel CPUs have support for recovering from some memory errors
  11 (``MCA recovery``). This requires the OS to declare a page "poisoned",
  12 kill the processes associated with it and avoid using it in the future.
  13
  14 This patchkit implements the necessary infrastructure in the VM.
  15
  16 To 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
  43 The code consists of a the high level handler in mm/memory-failure.c,
  44 a new page poison bit and various checks in the VM to handle poisoned
  45 pages.
  46
  47 The main target right now is KVM guests, but it works for all kinds
  48 of applications. KVM support requires a recent qemu-kvm release.
  49
  50 For the KVM use there was need for a new signal type so that
  51 KVM can inject the machine check into the guest with the proper
  52 address. This in theory allows other applications to handle
  53 memory failures too. The expection is that near all applications
  54 won't do that, but some very specialized ones might.
  55
  56 Failure recovery modes
  57 ======================
  58
  59 There are two (actually three) modes memory failure recovery can be in:
  60
  61 vm.memory_failure_recovery sysctl set to zero:
  62         All memory failures cause a panic. Do not attempt recovery.
  63         (on x86 this can be also affected by the tolerant level of the
  64         MCE subsystem)
  65
  66 early kill
  67         (can be controlled globally and per process)
  68         Send SIGBUS to the application as soon as the error is detected
  69         This allows applications who can process memory errors in a gentle
  70         way (e.g. drop affected object)
  71         This is the mode used by KVM qemu.
  72
  73 late kill
  74         Send SIGBUS when the application runs into the corrupted page.
  75         This is best for memory error unaware applications and default
  76         Note some pages are always handled as late kill.
  77
  78 User control
  79 ============
  80
  81 vm.memory_failure_recovery
  82         See sysctl.txt
  83
  84 vm.memory_failure_early_kill
  85         Enable early kill mode globally
  86
  87 PR_MCE_KILL
  88         Set early/late kill mode/revert to system default
  89
  90         arg1: PR_MCE_KILL_CLEAR:
  91                 Revert to system default
  92         arg1: PR_MCE_KILL_SET:
  93                 arg2 defines thread specific mode
  94
  95                 PR_MCE_KILL_EARLY:
  96                         Early kill
  97                 PR_MCE_KILL_LATE:
  98                         Late kill
  99                 PR_MCE_KILL_DEFAULT
 100                         Use system global default
 101
 102         Note that if you want to have a dedicated thread which handles
 103         the SIGBUS(BUS_MCEERR_AO) on behalf of the process, you should
 104         call prctl(PR_MCE_KILL_EARLY) on the designated thread. Otherwise,
 105         the SIGBUS is sent to the main thread.
 106
 107 PR_MCE_KILL_GET
 108         return current mode
 109
 110 Testing
 111 =======
 112
 113 * madvise(MADV_HWPOISON, ....) (as root) - Poison a page in the
 114   process for testing
 115
 116 * hwpoison-inject module through debugfs ``/sys/kernel/debug/hwpoison/``
 117
 118   corrupt-pfn
 119         Inject hwpoison fault at PFN echoed into this file. This does
 120         some early filtering to avoid corrupted unintended pages in test suites.
 121
 122   unpoison-pfn
 123         Software-unpoison page at PFN echoed into this file. This way
 124         a page can be reused again.  This only works for Linux
 125         injected failures, not for real memory failures.
 126
 127   Note these injection interfaces are not stable and might change between
 128   kernel versions
 129
 130   corrupt-filter-dev-major, corrupt-filter-dev-minor
 131         Only handle memory failures to pages associated with the file
 132         system defined by block device major/minor.  -1U is the
 133         wildcard value.  This should be only used for testing with
 134         artificial injection.
 135
 136   corrupt-filter-memcg
 137         Limit injection to pages owned by memgroup. Specified by inode
 138         number of the memcg.
 139
 140         Example::
 141
 142                 mkdir /sys/fs/cgroup/mem/hwpoison
 143
 144                 usemem -m 100 -s 1000 &
 145                 echo `jobs -p` > /sys/fs/cgroup/mem/hwpoison/tasks
 146
 147                 memcg_ino=$(ls -id /sys/fs/cgroup/mem/hwpoison | cut -f1 -d' ')
 148                 echo $memcg_ino > /debug/hwpoison/corrupt-filter-memcg
 149
 150                 page-types -p `pidof init`   --hwpoison  # shall do nothing
 151                 page-types -p `pidof usemem` --hwpoison  # poison its pages
 152
 153   corrupt-filter-flags-mask, corrupt-filter-flags-value
 154         When specified, only poison pages if ((page_flags & mask) ==
 155         value).  This allows stress testing of many kinds of
 156         pages. The page_flags are the same as in /proc/kpageflags. The
 157         flag bits are defined in include/linux/kernel-page-flags.h and
 158         documented in Documentation/admin-guide/mm/pagemap.rst
 159
 160 * Architecture specific MCE injector
 161
 162   x86 has mce-inject, mce-test
 163
 164   Some portable hwpoison test programs in mce-test, see below.
 165
 166 References
 167 ==========
 168
 169 http://halobates.de/mce-lc09-2.pdf
 170         Overview presentation from LinuxCon 09
 171
 172 git://git.kernel.org/pub/scm/utils/cpu/mce/mce-test.git
 173         Test suite (hwpoison specific portable tests in tsrc)
 174
 175 git://git.kernel.org/pub/scm/utils/cpu/mce/mce-inject.git
 176         x86 specific injector
 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