[PATCH v4 0/2] mm/process_vm_access: pidfd and nowait support for process_vm_readv/writev

[Alban Crequy <alban.crequy@gmail.com>]

This adds two flags to process_vm_readv/writev:

- PROCESS_VM_PIDFD: refer to the remote process via PID file descriptor
  instead of PID.
- PROCESS_VM_NOWAIT: do not block on IO if the memory access causes a
  page fault.

v3: https://lore.kernel.org/lkml/20260428122826.339550-1-alban.crequy@gmail.com/
v2: https://lore.kernel.org/lkml/20260408145436.843538-1-alban.crequy@gmail.com/
v1: https://lore.kernel.org/lkml/20251118132348.2415603-1-alban.crequy@gmail.com/
Sashiko review of v3: https://sashiko.dev/#/patchset/20260428122826.339550-1-alban.crequy@gmail.com
Sashiko review of v2: https://sashiko.dev/#/patchset/20260408145436.843538-1-alban.crequy@gmail.com

Changes since v3:
- Rename include/uapi/linux/process_vm.h to process_vm_access.h to
  match mm/process_vm_access.c (David Hildenbrand)
- Fix MAINTAINERS alphabetical sort order (David Hildenbrand)
- Document NOWAIT return value in commit message (David Hildenbrand)
- Document PIDFD EBADF error in commit message (David Hildenbrand)
- Add suggested man page update text in commit message
  (David Hildenbrand, Christian Brauner, Mike Rapoport)
- Keep (1UL << N) in UAPI header: BIT() is defined in vdso/bits.h
  which is not exported to userspace (David Hildenbrand)
- Add selftests for NOWAIT partial reads across resident and
  non-resident pages (single iovec and two iovecs)
- SKIP tests gracefully on kernels without flag support (Sashiko)
- Verify content of partial reads in selftests
- Rebase onto v7.1-rc3

Changes since v2:
- Fix ERR_PTR handling for pidfd_get_task(): use IS_ERR()/PTR_ERR()
  for the pidfd path, matching process_madvise() (Usama Arif, Sashiko)
- Add selftest for invalid pidfd (David Hildenbrand)
- Add selftest for invalid pid
- Remove hardcoded __NR_pidfd_open fallback, use <sys/syscall.h> (Sashiko)
- SKIP pidfd tests on kernels without pidfd_open (ENOSYS) (Sashiko)
- SKIP userfaultfd tests when unprivileged userfaultfd is disabled (EPERM) (Sashiko)
- Fault in test_data before NOWAIT tests to ensure page is resident (Sashiko)
- Add ksft_process_vm_readv.sh wrapper and run_vmtests.sh entry
  so the test runs in CI
- Rebase onto v7.1-rc1

Not addressed:
- uffd handler timeout causing test hang: kselftest_harness forks each
  test with a 30-second timeout, so an infinite hang cannot occur (Sashiko)
- 64-bit process reading 32-bit process high addresses: pre-existing
  concern in the existing process_vm_readv code, not introduced by this
  patch (David Laight)
- EAGAIN instead of short read/EFAULT for non-resident pages: short-read
  semantics are more informative and consistent with O_NONBLOCK
  conventions; the profiler use case does not intend to retry (Sashiko)
- mmap_read_trylock with NOWAIT: would expand scope significantly and is
  not how other FOLL_NOWAIT users in the kernel work (Sashiko)
- GFP_NOWAIT for page array allocation: the allocation is small and
  rarely blocks; not on the data path (Sashiko)
- volatile uint64_t cast on uint8_t test_data for page fault-in:
  alignment is not a concern in userspace programs (Sashiko)

Alban Crequy (2):
  mm/process_vm_access: pidfd and nowait support for
    process_vm_readv/writev
  selftests/mm: add tests for process_vm_readv flags

 MAINTAINERS                                   |   1 +
 include/uapi/linux/process_vm_access.h        |   9 +
 mm/process_vm_access.c                        |  34 +-
 tools/testing/selftests/mm/Makefile           |   2 +
 .../selftests/mm/ksft_process_vm_readv.sh     |   4 +
 tools/testing/selftests/mm/process_vm_readv.c | 573 ++++++++++++++++++
 tools/testing/selftests/mm/run_vmtests.sh     |   4 +
 7 files changed, 618 insertions(+), 9 deletions(-)
 create mode 100644 include/uapi/linux/process_vm_access.h
 create mode 100755 tools/testing/selftests/mm/ksft_process_vm_readv.sh
 create mode 100644 tools/testing/selftests/mm/process_vm_readv.c

-- 
2.45.0