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Subject[PATCH v5 00/10] userfaultfd: add minor fault handling for shmem

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:


- Picked up {Reviewed,Acked}-by's.
- Fix cleanup in error path in shmem_mcopy_atomic_pte(). [Hugh, Peter]
- Mention switching to lru_cache_add() in the commit message of 9/10. [Hugh]
- Split + reorder commits, so now we 1) implement the faulting path, 2)
implement the CONTINUE ioctl, and 3) advertise the feature. Squash the
documentation update into step (3). [Hugh, Peter]
- Reorder install_pte() cleanup to come before selftest changes. [Hugh]

- Fix handling of the shmem private mcopy case. Previously, I had (incorrectly)
assumed that !vma_is_anonymous() was equivalent to "the page will be in the
page cache". But, in this case we have an optimization where we allocate a new
*anonymous* page. So, use a new "bool page_in_cache" instead, which checks if
page->mapping is set. Correct several places with this new check. [Hugh]
- Fix calling mm_counter() before page_add_..._rmap(). [Hugh]
- When modifying shmem_mcopy_atomic_pte() to use the new install_pte() helper,
just use lru_cache_add_inactive_or_unevictable(), no need to branch and maybe
use lru_cache_add(). [Hugh]
- De-pluralize mcopy_atomic_install_pte(s). [Hugh]
- Make "writable" a bool, and initialize consistently. [Hugh]

- Picked up {Reviewed,Acked}-by's.
- Reorder commits: introduce CONTINUE before MINOR registration. [Hugh, Peter]
- Don't try to {unlock,put}_page an xarray value in shmem_getpage_gfp. [Hugh]
- Move enum mcopy_atomic_mode forward declare out of CONFIG_HUGETLB_PAGE. [Hugh]
- Keep mistakenly removed UFFD_USER_MODE_ONLY in selftest. [Peter]
- Cleanup context management in self test (make clear implicit, remove unneeded
return values now that we have err()). [Peter]
- Correct dst_pte argument to dst_pmd in shmem_mcopy_atomic_pte macro. [Hugh]
- Mention the new shmem support feature in documentation. [Hugh]

- 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]


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).
- Commit 5 advertises that the feature is now available since at this point it's
fully implemented.
- Commit 6 is a final cleanup, modifying an existing code path to re-use a new
helper we've introduced.
- Commits 7, 8, 9, 10 update the userfaultfd selftest to exercise the feature.

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 <>) 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."


Axel Rasmussen (10):
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/shmem: advertise shmem minor fault support
userfaultfd/shmem: modify shmem_mcopy_atomic_pte to use install_pte()
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

Documentation/admin-guide/mm/userfaultfd.rst | 3 +-
fs/userfaultfd.c | 6 +-
include/linux/hugetlb.h | 4 +-
include/linux/shmem_fs.h | 17 +-
include/linux/userfaultfd_k.h | 5 +
include/uapi/linux/userfaultfd.h | 7 +-
mm/hugetlb.c | 1 +
mm/memory.c | 8 +-
mm/shmem.c | 110 +++-----
mm/userfaultfd.c | 175 ++++++++----
tools/testing/selftests/vm/userfaultfd.c | 274 ++++++++++++-------
11 files changed, 360 insertions(+), 250 deletions(-)


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