Documentation/mm/hwpoison.rst

Source file repositories/reference/linux-study-clean/Documentation/mm/hwpoison.rst

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Documentation/mm/hwpoison.rst
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========
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 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.

Annotation

Implementation Notes