[PATCH v1 0/3] Userspace controls soft-offline HugeTLB pages

[Jiaqi Yan <jiaqiyan@google.com>]

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

-- 
2.45.1.288.g0e0cd299f1-goog