[PATCH v2 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. The 1st commit fixes an issue
in __folio_free_raw_hwp. The 2nd commit exports the functionality to
tell if a subpage inside a hugetlb hugepage is a raw HWPOISON page.
The 3rd commit teaches hugetlbfs_read_iter to return as many healthy
bytes as possible. The last commit properly tests this new feature.

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

Changelog

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

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2.41.0.162.gfafddb0af9-goog