[v4,00/26] userfaultfd-wp: Support shmem and hugetlbfs

Message

Peter Xu July 14, 2021, 10:20 p.m. UTC

This is v4 of uffd-wp shmem & hugetlbfs support, which completes uffd-wp as a
full feature.  It's based on v5.14-rc1.

The whole series can also be found online [1].  Nothing big really changed from
previous version.

One thing worth mentioning is in commit 22061a1ffabd we added a new single_page
parameter to zap_details from Hugh, then a few zap related patches need some
rebase around it.  There's also the new unmap_mapping_page() introduced, in
this series it will start to run with zap flag ZAP_FLAG_DROP_FILE_UFFD_WP, as
this function is used as the last phase to unmap shmem mappings when page being
e.g. truncated.  It actually even simplified a bit as I can drop the patch "mm:
Pass zap_flags into unmap_mapping_pages()" now.

Another thing to mention is I further modified (trivially) the test program
umap-apps [4] to allow backend storage to be run on a shmem file (it used to be
a e.g. XFS file, however as I noticed disk latency is a major bottleneck of
umapsort program especially when it's a HDD, shmem backend is much faster).
This should stress the kernel a bit more than before.

Full changelog listed below.

v4:
- Rebased to v5.14-rc1
- Collect r-b for Alistair
- Patch "mm/userfaultfd: Introduce special pte for unmapped file-backed mem"
  - make pte_swp_uffd_wp_special() return false for !HAVE_ARCH_USERFAULTFD_WP
    [Alistair]
- Patch "mm: Introduce zap_details.zap_flags"
  - Rename zap_check_mapping_skip to zap_skip_check_mapping [Alistair]
- Patch "mm: Pass zap_flags into unmap_mapping_pages()"
  - Dropped the patch because after commit 22061a1ffabd it's not needed anymore.
- Patch "mm/userfaultfd: Enable write protection for shmem & hugetlbfs"
  - Drop UFFD_FEATURE_WP_HUGETLBFS_SHMEM too if !CONFIG_HAVE_ARCH_USERFAULTFD_WP
- Patch "shmem/userfaultfd: Persist uffd-wp bit across zapping for file-backed":
  - Convert zap_pte_range() of device private to WARN_ON_ONCE() because it must
    not be a file-backed pte [Alistair]
  - Coordinate with new commit 22061a1ffabd ("mm/thp: unmap_mapping_page() to
    fix THP truncate_cleanup_page()"), add ZAP_FLAG_CHECK_MAPPING for new
    function unmap_mapping_page().

v3:
- Rebase to v5.13-rc3-mmots-2021-05-25-20-12
- Fix commit message and comment for patch "shmem/userfaultfd: Handle uffd-wp
  special pte in page fault handler", dropping all reference to FAULT_FLAG_UFFD_WP.
- Reworked patch "shmem/userfaultfd: Take care of UFFDIO_COPY_MODE_WP" after
  Axel's refactoring on uffdio-copy/continue.
- Added patch "mm/hugetlb: Introduce huge pte version of uffd-wp helpers", so
  that huge pte helpers are introduced in one patch.  Also add huge_pte_uffd_wp
  helper, which was missing previously.
- Added patch: "mm/pagemap: Recognize uffd-wp bit for shmem/hugetlbfs", to let
  pagemap uffd-wp bit work for shmem/hugetlbfs
- Added patch: "mm/shmem: Unconditionally set pte dirty in
  mfill_atomic_install_pte", to clean up dirty bit together in uffdio-copy

v2:
- Add R-bs
- Added patch "mm/hugetlb: Drop __unmap_hugepage_range definition from
  hugetlb.h" as noticed/suggested by Mike Kravets
- Fix commit message of patch "hugetlb/userfaultfd: Only drop uffd-wp special
  pte if required" [MikeK]
- Removing comments for fields in zap_details since they're either incorrect or
  not helping [Matthew]
- Rephrase commit message in patch "hugetlb/userfaultfd: Take care of
  UFFDIO_COPY_MODE_WP" to explain better on why set dirty bit for UFFDIO_COPY
  in hugetlbfs [MikeK]
- Don't emulate READ for uffd-wp-special on both shmem & hugetlbfs.
- Drop FAULT_FLAG_UFFD_WP flag, by checking vmf->orig_pte directly against
  pte_swp_uffd_wp_special()
- Fix race condition of page fault handling on uffd-wp-special [Mike]

About Swap Special PTE
======================

In short, the so-called "swap special pte" in this patchset is a new type of
pte that doesn't exist in the past, but it got used initially in this series in
file-backed memories.  It is used to persist information even if the ptes got
dropped meanwhile when the page cache still existed.  For example, when
splitting a file-backed huge pmd, we could be simply dropping the pmd entry
then wait until another fault coming.  It's okay in the past since all
information in the pte can be retained from the page cache when the next page
fault triggers.  However in this case, uffd-wp is per-pte information which
cannot be kept in page cache, so that information needs to be maintained
somehow still in the pgtable entry, even if the pgtable entry is going to be
dropped.  Here instead of replacing with a none entry, we used the "swap
special pte".  Then when the next page fault triggers, we can observe orig_pte
to retain this information.

I'm copy-pasting some commit message from the patch "mm/swap: Introduce the
idea of special swap ptes", where it tried to explain this pte in another angle:

    We used to have special swap entries, like migration entries, hw-poison
    entries, device private entries, etc.

    Those "special swap entries" reside in the range that they need to be at least
    swap entries first, and their types are decided by swp_type(entry).

    This patch introduces another idea called "special swap ptes".

    It's very easy to get confused against "special swap entries", but a speical
    swap pte should never contain a swap entry at all.  It means, it's illegal to
    call pte_to_swp_entry() upon a special swap pte.

    Make the uffd-wp special pte to be the first special swap pte.

    Before this patch, is_swap_pte()==true means one of the below:

       (a.1) The pte has a normal swap entry (non_swap_entry()==false).  For
             example, when an anonymous page got swapped out.

       (a.2) The pte has a special swap entry (non_swap_entry()==true).  For
             example, a migration entry, a hw-poison entry, etc.

    After this patch, is_swap_pte()==true means one of the below, where case (b) is
    added:

     (a) The pte contains a swap entry.

       (a.1) The pte has a normal swap entry (non_swap_entry()==false).  For
             example, when an anonymous page got swapped out.

       (a.2) The pte has a special swap entry (non_swap_entry()==true).  For
             example, a migration entry, a hw-poison entry, etc.

     (b) The pte does not contain a swap entry at all (so it cannot be passed
         into pte_to_swp_entry()).  For example, uffd-wp special swap pte.

Hugetlbfs needs similar thing because it's also file-backed.  I directly reused
the same special pte there, though the shmem/hugetlb change on supporting this
new pte is different since they don't share code path a lot.

Patch layout
============

Part (1): Shmem support, this is where the special swap pte is introduced.
Some zap rework is needed within the process:

  mm/shmem: Unconditionally set pte dirty in mfill_atomic_install_pte
  shmem/userfaultfd: Take care of UFFDIO_COPY_MODE_WP
  mm: Clear vmf->pte after pte_unmap_same() returns
  mm/userfaultfd: Introduce special pte for unmapped file-backed mem
  mm/swap: Introduce the idea of special swap ptes
  shmem/userfaultfd: Handle uffd-wp special pte in page fault handler
  mm: Drop first_index/last_index in zap_details
  mm: Introduce zap_details.zap_flags
  mm: Introduce ZAP_FLAG_SKIP_SWAP
  shmem/userfaultfd: Persist uffd-wp bit across zapping for file-backed
  shmem/userfaultfd: Allow wr-protect none pte for file-backed mem
  shmem/userfaultfd: Allows file-back mem to be uffd wr-protected on thps
  shmem/userfaultfd: Handle the left-overed special swap ptes
  shmem/userfaultfd: Pass over uffd-wp special swap pte when fork()

Part (2): Hugetlb supportdisable huge pmd sharing for uffd-wp patches have been
merged.  The rest is the changes required to teach hugetlbfs understand the
special swap pte too that introduced with the uffd-wp change:

  mm/hugetlb: Drop __unmap_hugepage_range definition from hugetlb.h
  mm/hugetlb: Introduce huge pte version of uffd-wp helpers
  hugetlb/userfaultfd: Hook page faults for uffd write protection
  hugetlb/userfaultfd: Take care of UFFDIO_COPY_MODE_WP
  hugetlb/userfaultfd: Handle UFFDIO_WRITEPROTECT
  mm/hugetlb: Introduce huge version of special swap pte helpers
  hugetlb/userfaultfd: Handle uffd-wp special pte in hugetlb pf handler
  hugetlb/userfaultfd: Allow wr-protect none ptes
  hugetlb/userfaultfd: Only drop uffd-wp special pte if required

Part (3): Enable both features in code and test (plus pagemap support)

  mm/pagemap: Recognize uffd-wp bit for shmem/hugetlbfs
  userfaultfd: Enable write protection for shmem & hugetlbfs
  userfaultfd/selftests: Enable uffd-wp for shmem/hugetlbfs

Tests
=====

I've tested it using either userfaultfd kselftest program, but also with
umapsort [2] which should be even stricter.  Tested page swapping in/out during
umapsort.

If anyone would like to try umapsort, need to use an extremely hacked version
of umap library [3], because by default umap only supports anonymous.  So to
test it we need to build [3] then [2].

Any comment would be greatly welcomed.  Thanks,

[1] https://github.com/xzpeter/linux/tree/uffd-wp-shmem-hugetlbfs
[2] https://github.com/xzpeter/umap-apps/tree/peter
[3] https://github.com/xzpeter/umap/tree/peter-shmem-hugetlbfs
[4] https://github.com/xzpeter/umap-apps/commit/b0c2c7b1cd9dcb6835e7c59d02ece1f6b7f1ea01

Peter Xu (26):
  mm/shmem: Unconditionally set pte dirty in mfill_atomic_install_pte
  shmem/userfaultfd: Take care of UFFDIO_COPY_MODE_WP
  mm: Clear vmf->pte after pte_unmap_same() returns
  mm/userfaultfd: Introduce special pte for unmapped file-backed mem
  mm/swap: Introduce the idea of special swap ptes
  shmem/userfaultfd: Handle uffd-wp special pte in page fault handler
  mm: Drop first_index/last_index in zap_details
  mm: Introduce zap_details.zap_flags
  mm: Introduce ZAP_FLAG_SKIP_SWAP
  shmem/userfaultfd: Persist uffd-wp bit across zapping for file-backed
  shmem/userfaultfd: Allow wr-protect none pte for file-backed mem
  shmem/userfaultfd: Allows file-back mem to be uffd wr-protected on
    thps
  shmem/userfaultfd: Handle the left-overed special swap ptes
  shmem/userfaultfd: Pass over uffd-wp special swap pte when fork()
  mm/hugetlb: Drop __unmap_hugepage_range definition from hugetlb.h
  mm/hugetlb: Introduce huge pte version of uffd-wp helpers
  hugetlb/userfaultfd: Hook page faults for uffd write protection
  hugetlb/userfaultfd: Take care of UFFDIO_COPY_MODE_WP
  hugetlb/userfaultfd: Handle UFFDIO_WRITEPROTECT
  mm/hugetlb: Introduce huge version of special swap pte helpers
  hugetlb/userfaultfd: Handle uffd-wp special pte in hugetlb pf handler
  hugetlb/userfaultfd: Allow wr-protect none ptes
  hugetlb/userfaultfd: Only drop uffd-wp special pte if required
  mm/pagemap: Recognize uffd-wp bit for shmem/hugetlbfs
  mm/userfaultfd: Enable write protection for shmem & hugetlbfs
  userfaultfd/selftests: Enable uffd-wp for shmem/hugetlbfs

 arch/arm64/kernel/mte.c                  |   2 +-
 arch/x86/include/asm/pgtable.h           |  28 +++
 fs/hugetlbfs/inode.c                     |  15 +-
 fs/proc/task_mmu.c                       |  21 +-
 fs/userfaultfd.c                         |  41 ++--
 include/asm-generic/hugetlb.h            |  15 ++
 include/asm-generic/pgtable_uffd.h       |   3 +
 include/linux/hugetlb.h                  |  30 ++-
 include/linux/mm.h                       |  44 +++-
 include/linux/mm_inline.h                |  42 ++++
 include/linux/shmem_fs.h                 |   4 +-
 include/linux/swapops.h                  |  39 +++-
 include/linux/userfaultfd_k.h            |  49 +++++
 include/uapi/linux/userfaultfd.h         |  10 +-
 mm/gup.c                                 |   2 +-
 mm/hmm.c                                 |   2 +-
 mm/hugetlb.c                             | 160 ++++++++++++---
 mm/khugepaged.c                          |  11 +-
 mm/madvise.c                             |   4 +-
 mm/memcontrol.c                          |   2 +-
 mm/memory.c                              | 244 +++++++++++++++++------
 mm/migrate.c                             |   4 +-
 mm/mincore.c                             |   2 +-
 mm/mprotect.c                            |  63 +++++-
 mm/mremap.c                              |   2 +-
 mm/page_vma_mapped.c                     |   6 +-
 mm/rmap.c                                |   8 +
 mm/shmem.c                               |   5 +-
 mm/swapfile.c                            |   2 +-
 mm/userfaultfd.c                         |  73 +++++--
 tools/testing/selftests/vm/userfaultfd.c |   9 +-
 31 files changed, 756 insertions(+), 186 deletions(-)