185 lines
5.9 KiB
ReStructuredText
185 lines
5.9 KiB
ReStructuredText
.. hwpoison:
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========
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hwpoison
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========
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What is hwpoison?
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=================
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Upcoming Intel CPUs have support for recovering from some memory errors
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(``MCA recovery``). This requires the OS to declare a page "poisoned",
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kill the processes associated with it and avoid using it in the future.
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This patchkit implements the necessary infrastructure in the VM.
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To quote the overview comment::
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High level machine check handler. Handles pages reported by the
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hardware as being corrupted usually due to a 2bit ECC memory or cache
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failure.
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This focusses on pages detected as corrupted in the background.
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When the current CPU tries to consume corruption the currently
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running process can just be killed directly instead. This implies
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that if the error cannot be handled for some reason it's safe to
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just ignore it because no corruption has been consumed yet. Instead
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when that happens another machine check will happen.
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Handles page cache pages in various states. The tricky part
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here is that we can access any page asynchronous to other VM
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users, because memory failures could happen anytime and anywhere,
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possibly violating some of their assumptions. This is why this code
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has to be extremely careful. Generally it tries to use normal locking
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rules, as in get the standard locks, even if that means the
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error handling takes potentially a long time.
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Some of the operations here are somewhat inefficient and have non
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linear algorithmic complexity, because the data structures have not
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been optimized for this case. This is in particular the case
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for the mapping from a vma to a process. Since this case is expected
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to be rare we hope we can get away with this.
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The code consists of a the high level handler in mm/memory-failure.c,
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a new page poison bit and various checks in the VM to handle poisoned
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pages.
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The main target right now is KVM guests, but it works for all kinds
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of applications. KVM support requires a recent qemu-kvm release.
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For the KVM use there was need for a new signal type so that
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KVM can inject the machine check into the guest with the proper
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address. This in theory allows other applications to handle
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memory failures too. The expection is that near all applications
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won't do that, but some very specialized ones might.
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Failure recovery modes
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======================
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There are two (actually three) modes memory failure recovery can be in:
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vm.memory_failure_recovery sysctl set to zero:
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All memory failures cause a panic. Do not attempt recovery.
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early kill
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(can be controlled globally and per process)
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Send SIGBUS to the application as soon as the error is detected
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This allows applications who can process memory errors in a gentle
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way (e.g. drop affected object)
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This is the mode used by KVM qemu.
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late kill
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Send SIGBUS when the application runs into the corrupted page.
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This is best for memory error unaware applications and default
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Note some pages are always handled as late kill.
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User control
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============
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vm.memory_failure_recovery
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See sysctl.txt
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vm.memory_failure_early_kill
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Enable early kill mode globally
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PR_MCE_KILL
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Set early/late kill mode/revert to system default
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arg1: PR_MCE_KILL_CLEAR:
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Revert to system default
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arg1: PR_MCE_KILL_SET:
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arg2 defines thread specific mode
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PR_MCE_KILL_EARLY:
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Early kill
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PR_MCE_KILL_LATE:
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Late kill
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PR_MCE_KILL_DEFAULT
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Use system global default
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Note that if you want to have a dedicated thread which handles
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the SIGBUS(BUS_MCEERR_AO) on behalf of the process, you should
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call prctl(PR_MCE_KILL_EARLY) on the designated thread. Otherwise,
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the SIGBUS is sent to the main thread.
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PR_MCE_KILL_GET
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return current mode
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Testing
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=======
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* madvise(MADV_HWPOISON, ....) (as root) - Poison a page in the
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process for testing
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* hwpoison-inject module through debugfs ``/sys/kernel/debug/hwpoison/``
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corrupt-pfn
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Inject hwpoison fault at PFN echoed into this file. This does
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some early filtering to avoid corrupted unintended pages in test suites.
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unpoison-pfn
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Software-unpoison page at PFN echoed into this file. This way
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a page can be reused again. This only works for Linux
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injected failures, not for real memory failures. Once any hardware
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memory failure happens, this feature is disabled.
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Note these injection interfaces are not stable and might change between
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kernel versions
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corrupt-filter-dev-major, corrupt-filter-dev-minor
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Only handle memory failures to pages associated with the file
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system defined by block device major/minor. -1U is the
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wildcard value. This should be only used for testing with
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artificial injection.
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corrupt-filter-memcg
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Limit injection to pages owned by memgroup. Specified by inode
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number of the memcg.
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Example::
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mkdir /sys/fs/cgroup/mem/hwpoison
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usemem -m 100 -s 1000 &
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echo `jobs -p` > /sys/fs/cgroup/mem/hwpoison/tasks
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memcg_ino=$(ls -id /sys/fs/cgroup/mem/hwpoison | cut -f1 -d' ')
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echo $memcg_ino > /debug/hwpoison/corrupt-filter-memcg
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page-types -p `pidof init` --hwpoison # shall do nothing
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page-types -p `pidof usemem` --hwpoison # poison its pages
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corrupt-filter-flags-mask, corrupt-filter-flags-value
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When specified, only poison pages if ((page_flags & mask) ==
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value). This allows stress testing of many kinds of
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pages. The page_flags are the same as in /proc/kpageflags. The
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flag bits are defined in include/linux/kernel-page-flags.h and
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documented in Documentation/admin-guide/mm/pagemap.rst
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* Architecture specific MCE injector
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x86 has mce-inject, mce-test
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Some portable hwpoison test programs in mce-test, see below.
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References
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==========
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http://halobates.de/mce-lc09-2.pdf
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Overview presentation from LinuxCon 09
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git://git.kernel.org/pub/scm/utils/cpu/mce/mce-test.git
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Test suite (hwpoison specific portable tests in tsrc)
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git://git.kernel.org/pub/scm/utils/cpu/mce/mce-inject.git
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x86 specific injector
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Limitations
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===========
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- Not all page types are supported and never will. Most kernel internal
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objects cannot be recovered, only LRU pages for now.
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---
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Andi Kleen, Oct 2009
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