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    Subject[PATCH v2 0/9] userfaultfd: add minor fault handling for shmem
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    Base
    ====

    This series is based on (and therefore should apply cleanly to) the tag
    "v5.12-rc7-mmots-2021-04-11-20-49", additionally with Peter's selftest cleanup
    series applied *first*:

    https://lore.kernel.org/patchwork/cover/1412450/

    Changelog
    =========

    v1->v2:
    - Pick up Reviewed-by's.
    - Don't swapin page when a minor fault occurs. Notice that it needs to be
    swapped in, and just immediately fire the minor fault. Let a future CONTINUE
    deal with swapping in the page. [Peter]
    - Clarify comment about i_size checks in mm/userfaultfd.c. [Peter]
    - Only forward declare once (out of #ifdef) in hugetlb.h. [Peter]

    Changes since [2]:
    - Squash the fixes ([2]) in with the original series ([1]). This makes reviewing
    easier, as we no longer have to sift through deltas undoing what we had done
    before. [Hugh, Peter]
    - Modify shmem_mcopy_atomic_pte() to use the new mcopy_atomic_install_ptes()
    helper, reducing code duplication. [Hugh]
    - Properly trigger handle_userfault() in the shmem_swapin_page() case. [Hugh]
    - Use shmem_getpage() instead of find_lock_page() to lookup the existing page in
    for continue. This properly deals with swapped-out pages. [Hugh]
    - Unconditionally pte_mkdirty() for anon memory (as before). [Peter]
    - Don't include userfaultfd_k.h in either hugetlb.h or shmem_fs.h. [Hugh]
    - Add comment for UFFD_FEATURE_MINOR_SHMEM (to match _HUGETLBFS). [Hugh]
    - Fix some small cleanup issues (parens, reworded conditionals, reduced plumbing
    of some parameters, simplify labels/gotos, ...). [Hugh, Peter]

    Overview
    ========

    See the series which added minor faults for hugetlbfs [3] for a detailed
    overview of minor fault handling in general. This series adds the same support
    for shmem-backed areas.

    This series is structured as follows:

    - Commits 1 and 2 are cleanups.
    - Commits 3 and 4 implement the new feature (minor fault handling for shmem).
    - Commits 5, 6, 7, 8 update the userfaultfd selftest to exercise the feature.
    - Commit 9 is one final cleanup, modifying an existing code path to re-use a new
    helper we've introduced. We rely on the selftest to show that this change
    doesn't break anything.

    Use Case
    ========

    In some cases it is useful to have VM memory backed by tmpfs instead of
    hugetlbfs. So, this feature will be used to support the same VM live migration
    use case described in my original series.

    Additionally, Android folks (Lokesh Gidra <lokeshgidra@google.com>) hope to
    optimize the Android Runtime garbage collector using this feature:

    "The plan is to use userfaultfd for concurrently compacting the heap. With
    this feature, the heap can be shared-mapped at another location where the
    GC-thread(s) could continue the compaction operation without the need to
    invoke userfault ioctl(UFFDIO_COPY) each time. OTOH, if and when Java threads
    get faults on the heap, UFFDIO_CONTINUE can be used to resume execution.
    Furthermore, this feature enables updating references in the 'non-moving'
    portion of the heap efficiently. Without this feature, uneccessary page
    copying (ioctl(UFFDIO_COPY)) would be required."

    [1] https://lore.kernel.org/patchwork/cover/1388144/
    [2] https://lore.kernel.org/patchwork/patch/1408161/
    [3] https://lore.kernel.org/linux-fsdevel/20210301222728.176417-1-axelrasmussen@google.com/T/#t

    Axel Rasmussen (9):
    userfaultfd/hugetlbfs: avoid including userfaultfd_k.h in hugetlb.h
    userfaultfd/shmem: combine shmem_{mcopy_atomic,mfill_zeropage}_pte
    userfaultfd/shmem: support minor fault registration for shmem
    userfaultfd/shmem: support UFFDIO_CONTINUE for shmem
    userfaultfd/selftests: use memfd_create for shmem test type
    userfaultfd/selftests: create alias mappings in the shmem test
    userfaultfd/selftests: reinitialize test context in each test
    userfaultfd/selftests: exercise minor fault handling shmem support
    userfaultfd/shmem: modify shmem_mcopy_atomic_pte to use install_ptes

    fs/userfaultfd.c | 6 +-
    include/linux/hugetlb.h | 4 +-
    include/linux/shmem_fs.h | 15 +-
    include/linux/userfaultfd_k.h | 5 +
    include/uapi/linux/userfaultfd.h | 7 +-
    mm/hugetlb.c | 1 +
    mm/memory.c | 8 +-
    mm/shmem.c | 112 +++------
    mm/userfaultfd.c | 183 ++++++++++-----
    tools/testing/selftests/vm/userfaultfd.c | 280 +++++++++++++++--------
    10 files changed, 377 insertions(+), 244 deletions(-)

    --
    2.31.1.295.g9ea45b61b8-goog

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