Correctable memory errors are very common on servers with large amount of memory, and are corrected by ECC, but with two pain points to users: 1. Correction usually happens on the fly and adds latency overhead 2. Not-fully-proved theory states excessive correctable memory errors can develop into uncorrectable memory error.
Soft offline is kernel's additional solution for memory pages having (excessive) corrected memory errors. Impacted page is migrated to healthy page if it is in use, then the original page is discarded for any future use.
The actual policy on whether (and when) to soft offline should be maintained by userspace, especially in case of HugeTLB hugepages. Soft-offline dissolves a hugepage, either in-use or free, into chunks of 4K pages, reducing HugeTLB pool capacity by 1 hugepage. If userspace has not acknowledged such behavior, it may be surprised when later mmap hugepages MAP_FAILED due to lack of hugepages. In addition, discarding the entire 1G memory page only because of corrected memory errors sounds very costly and kernel better not doing under the hood. But today there are at least 2 such cases: 1. GHES driver sees both GHES_SEV_CORRECTED and CPER_SEC_ERROR_THRESHOLD_EXCEEDED after parsing CPER. 2. RAS Correctable Errors Collector counts correctable errors per PFN and when the counter for a PFN reaches threshold In both cases, userspace has no control of the soft offline performed by kernel's memory failure recovery.
This patch series give userspace the control of soft-offlining HugeTLB pages: kernel only soft offlines hugepage if userspace has opt-ed in for that specific hugepage size, and exposed to userspace by a new sysfs entry called softoffline_corrected_errors under /sys/kernel/mm/hugepages/hugepages-${size}kB directory: * When softoffline_corrected_errors=0, skip soft offlining for all hugepages of size ${size}kB. * When softoffline_corrected_errors=1, soft offline as before this patch series. By default softoffline_corrected_errors is 1.
This patch set is based at commit a52b4f11a2e1 ("selftest mm/mseal read-only elf memory segment").
Jiaqi Yan (3): mm/memory-failure: userspace controls soft-offlining hugetlb pages selftest/mm: test softoffline_corrected_errors behaviors docs: hugetlbpage.rst: add softoffline_corrected_errors
Documentation/admin-guide/mm/hugetlbpage.rst | 15 +- include/linux/hugetlb.h | 17 ++ mm/hugetlb.c | 34 +++ mm/memory-failure.c | 7 + tools/testing/selftests/mm/.gitignore | 1 + tools/testing/selftests/mm/Makefile | 1 + .../selftests/mm/hugetlb-soft-offline.c | 262 ++++++++++++++++++ tools/testing/selftests/mm/run_vmtests.sh | 4 + 8 files changed, 340 insertions(+), 1 deletion(-) create mode 100644 tools/testing/selftests/mm/hugetlb-soft-offline.c