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.
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. (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.
Enable early kill mode globally
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
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.
return current mode
madvise(MADV_HWPOISON, ….) (as root) - Poison a page in the process for testing
hwpoison-inject module through debugfs
Inject hwpoison fault at PFN echoed into this file. This does some early filtering to avoid corrupted unintended pages in test suites.
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.
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.
Limit injection to pages owned by memgroup. Specified by inode number of the memcg.
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 Examining Process Page Tables
Architecture specific MCE injector
x86 has mce-inject, mce-test
Some portable hwpoison test programs in mce-test, see below.
Overview presentation from LinuxCon 09
Test suite (hwpoison specific portable tests in tsrc)
x86 specific injector
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