========
hwpoison
========

What is hwpoison?
=================

Upcoming Intel CPUs have support for recovering from some memory errors
(``MCA recovery``). This requires the OS to declare a page "poisoned",
kill the processes associated with it and avoid using it in the future.

This patchkit implements the necessary infrastructure in the VM.

To quote the overview comment::

	High level machine check handler. Handles pages reported by the
	hardware as being corrupted usually due to a 2bit ECC memory or cache
	failure.

	This focusses on pages detected as corrupted in the background.
	When the current CPU tries to consume corruption the currently
	running process can just be killed directly instead. This implies
	that if the error cannot be handled for some reason it's safe to
	just ignore it because no corruption has been consumed yet. Instead
	when that happens another machine check will happen.

	Handles page cache pages in various states. The tricky part
	here is that we can access any page asynchronous to other VM
	users, because memory failures could happen anytime and anywhere,
	possibly violating some of their assumptions. This is why this code
	has to be extremely careful. Generally it tries to use normal locking
	rules, as in get the standard locks, even if that means the
	error handling takes potentially a long time.

	Some of the operations here are somewhat inefficient and have non
	linear algorithmic complexity, because the data structures have not
	been optimized for this case. This is in particular the case
	for the mapping from a vma to a process. Since this case is expected
	to be rare we hope we can get away with this.

The code consists of a the high level handler in mm/memory-failure.c,
a new page poison bit and various checks in the VM to handle poisoned
pages.

The main target right now is KVM guests, but it works for all kinds
of applications. KVM support requires a recent qemu-kvm release.

For the KVM use there was need for a new signal type so that
KVM can inject the machine check into the guest with the proper
address. This in theory allows other applications to handle
memory failures too. The expectation is that most applications
won't do that, but some very specialized ones might.

Failure recovery modes
======================

There are two (actually three) modes memory failure recovery can be in:

vm.memory_failure_recovery sysctl set to zero:
	All memory failures cause a panic. Do not attempt recovery.

early kill
	(can be controlled globally and per process)
	Send SIGBUS to the application as soon as the error is detected
	This allows applications who can process memory errors in a gentle
	way (e.g. drop affected object)
	This is the mode used by KVM qemu.

late kill
	Send SIGBUS when the application runs into the corrupted page.
	This is best for memory error unaware applications and default
	Note some pages are always handled as late kill.

User control
============

vm.memory_failure_recovery
	See sysctl.txt

vm.memory_failure_early_kill
	Enable early kill mode globally

PR_MCE_KILL
	Set early/late kill mode/revert to system default

	arg1: PR_MCE_KILL_CLEAR:
		Revert to system default
	arg1: PR_MCE_KILL_SET:
		arg2 defines thread specific mode

		PR_MCE_KILL_EARLY:
			Early kill
		PR_MCE_KILL_LATE:
			Late kill
		PR_MCE_KILL_DEFAULT
			Use system global default

	Note that if you want to have a dedicated thread which handles
	the SIGBUS(BUS_MCEERR_AO) on behalf of the process, you should
	call prctl(PR_MCE_KILL_EARLY) on the designated thread. Otherwise,
	the SIGBUS is sent to the main thread.

PR_MCE_KILL_GET
	return current mode

Testing
=======

* madvise(MADV_HWPOISON, ....) (as root) - Poison a page in the
  process for testing

* hwpoison-inject module through debugfs ``/sys/kernel/debug/hwpoison/``

  corrupt-pfn
	Inject hwpoison fault at PFN echoed into this file. This does
	some early filtering to avoid corrupted unintended pages in test suites.

  unpoison-pfn
	Software-unpoison page at PFN echoed into this file. This way
	a page can be reused again.  This only works for Linux
	injected failures, not for real memory failures. Once any hardware
	memory failure happens, this feature is disabled.

  Note these injection interfaces are not stable and might change between
  kernel versions

  corrupt-filter-dev-major, corrupt-filter-dev-minor
	Only handle memory failures to pages associated with the file
	system defined by block device major/minor.  -1U is the
	wildcard value.  This should be only used for testing with
	artificial injection.

  corrupt-filter-memcg
	Limit injection to pages owned by memgroup. Specified by inode
	number of the memcg.

	Example::

		mkdir /sys/fs/cgroup/mem/hwpoison

	        usemem -m 100 -s 1000 &
		echo `jobs -p` > /sys/fs/cgroup/mem/hwpoison/tasks

		memcg_ino=$(ls -id /sys/fs/cgroup/mem/hwpoison | cut -f1 -d' ')
		echo $memcg_ino > /debug/hwpoison/corrupt-filter-memcg

		page-types -p `pidof init`   --hwpoison  # shall do nothing
		page-types -p `pidof usemem` --hwpoison  # poison its pages

  corrupt-filter-flags-mask, corrupt-filter-flags-value
	When specified, only poison pages if ((page_flags & mask) ==
	value).  This allows stress testing of many kinds of
	pages. The page_flags are the same as in /proc/kpageflags. The
	flag bits are defined in include/linux/kernel-page-flags.h and
	documented in Documentation/admin-guide/mm/pagemap.rst

* Architecture specific MCE injector

  x86 has mce-inject, mce-test

  Some portable hwpoison test programs in mce-test, see below.

References
==========

http://halobates.de/mce-lc09-2.pdf
	Overview presentation from LinuxCon 09

git://git.kernel.org/pub/scm/utils/cpu/mce/mce-test.git
	Test suite (hwpoison specific portable tests in tsrc)

git://git.kernel.org/pub/scm/utils/cpu/mce/mce-inject.git
	x86 specific injector


Limitations
===========
- Not all page types are supported and never will. Most kernel internal
  objects cannot be recovered, only LRU pages for now.

---
Andi Kleen, Oct 2009