HWPOISON: Add brief hwpoison description to Documentation

Signed-off-by: Andi Kleen <ak@linux.intel.com>

diff --git a/Documentation/vm/hwpoison.txt b/Documentation/vm/hwpoison.txt
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+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 expection is that near all applications
+won't do that, but some very specialized ones might.
+
+---
+
+There are two (actually three) modi memory failure recovery can be in:
+
+vm.memory_failure_recovery sysctl set to zero:
+       All memory failures cause a panic. Do not attempt recovery.
+       (on x86 this can be also affected by the tolerant level of the
+       MCE subsystem)
+
+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
+PR_MCE_KILL_GET
+       return current mode
+
+
+---
+
+Testing:
+
+madvise(MADV_POISON, ....)
+       (as root)
+       Poison a page in the process for testing
+
+
+hwpoison-inject module through debugfs
+       /sys/debug/hwpoison/corrupt-pfn
+
+Inject hwpoison fault at PFN echoed into this file
+
+
+Architecture specific MCE injector
+
+x86 has mce-inject, mce-test
+
+Some portable hwpoison test programs in mce-test, see blow.
+
+---
+
+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.
+- Right now hugepage support is missing.
+
+---
+Andi Kleen, Oct 2009
+