[PATCH v3 0/4] Improve hugetlbfs read on HWPOISON hugepages

[Jiaqi Yan <jiaqiyan@google.com>]

Today when hardware memory is corrupted in a hugetlb hugepage,
kernel leaves the hugepage in pagecache [1]; otherwise future mmap or
read will suject to silent data corruption. This is implemented by
returning -EIO from hugetlb_read_iter immediately if the hugepage has
HWPOISON flag set.

Since memory_failure already tracks the raw HWPOISON subpages in a
hugepage, a natural improvement is possible: if userspace only asks for
healthy subpages in the pagecache, kernel can return these data.

This patchset implements this improvement. It consist of three parts.
The 1st commit exports the functionality to tell if a subpage inside a
hugetlb hugepage is a raw HWPOISON page. The 2nd commit teaches
hugetlbfs_read_iter to return as many healthy bytes as possible.
The 3rd commit properly tests this new feature.

[1] commit 8625147cafaa ("hugetlbfs: don't delete error page from pagecache")

Changelog

v2 => v3
* Updates commit messages for future reader to know background and
  code details
* v3 is based on commit 5bb367dca2b9 ("Merge branch 'master' into
  mm-stable")

v1 => v2
* __folio_free_raw_hwp deletes all entries in raw_hwp_list before it
  traverses and frees raw_hwp_page.
* find_raw_hwp_page => __is_raw_hwp_subpage and __is_raw_hwp_subpage
  only returns bool instead of a raw_hwp_page entry.
* is_raw_hwp_subpage holds hugetlb_lock while checking
  __is_raw_hwp_subpage.
* No need to do folio_lock in adjust_range_hwpoison.
* v2 is based on commit a6e79df92e4a ("mm/gup: disallow FOLL_LONGTERM
  GUP-fast writing to file-backed mappings")

Jiaqi Yan (4):
  mm/hwpoison: delete all entries before traversal in
    __folio_free_raw_hwp
  mm/hwpoison: check if a subpage of a hugetlb folio is raw HWPOISON
  hugetlbfs: improve read HWPOISON hugepage
  selftests/mm: add tests for HWPOISON hugetlbfs read

 fs/hugetlbfs/inode.c                          |  58 +++-
 include/linux/hugetlb.h                       |  19 ++
 include/linux/mm.h                            |   7 +
 mm/hugetlb.c                                  |  10 +
 mm/memory-failure.c                           |  42 ++-
 tools/testing/selftests/mm/.gitignore         |   1 +
 tools/testing/selftests/mm/Makefile           |   1 +
 .../selftests/mm/hugetlb-read-hwpoison.c      | 322 ++++++++++++++++++
 8 files changed, 439 insertions(+), 21 deletions(-)
 create mode 100644 tools/testing/selftests/mm/hugetlb-read-hwpoison.c

-- 
2.41.0.255.g8b1d071c50-goog