[PATCH 00/10] mm/mremap: permit mremap() move of multiple VMAs

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

Historically we've made it a uAPI requirement that mremap() may only
operate on a single VMA at a time.

For instances where VMAs need to be resized, this makes sense, as it
becomes very difficult to determine what a user actually wants should they
indicate a desire to expand or shrink the size of multiple VMAs (truncate?
Adjust sizes individually? Some other strategy?).

However, in instances where a user is moving VMAs, it is restrictive to
disallow this.

This is especially the case when anonymous mapping remap may or may not be
mergeable depending on whether VMAs have or have not been faulted due to
anon_vma assignment and folio index alignment with vma->vm_pgoff.

Often this can result in surprising impact where a moved region is faulted,
then moved back and a user fails to observe a merge from otherwise
compatible, adjacent VMAs.

This change allows such cases to work without the user having to be
cognizant of whether a prior mremap() move or other VMA operations has
resulted in VMA fragmentation.

In order to do this, this series performs a large amount of refactoring,
most pertinently - grouping sanity checks together, separately those that
check input parameters and those relating to VMAs.

we also simplify the post-mmap lock drop processing for uffd and mlock()'d
VMAs.

With this done, we can then fairly straightforwardly implement this
functionality.

This works exclusively for mremap() invocations which specify
MREMAP_FIXED. It is not compatible with VMAs which use userfaultfd, as the
notification of the userland fault handler would require us to drop the
mmap lock.

The input and output addresses ranges must not overlap. We carefully
account for moves which would result in VMA merges or would otherwise
result in VMA iterator invalidation.

Lorenzo Stoakes (10):
  mm/mremap: perform some simple cleanups
  mm/mremap: refactor initial parameter sanity checks
  mm/mremap: put VMA check and prep logic into helper function
  mm/mremap: cleanup post-processing stage of mremap
  mm/mremap: use an explicit uffd failure path for mremap
  mm/mremap: check remap conditions earlier
  mm/mremap: move remap_is_valid() into check_prep_vma()
  mm/mremap: clean up mlock populate behaviour
  mm/mremap: permit mremap() move of multiple VMAs
  tools/testing/selftests: extend mremap_test to test multi-VMA mremap

 fs/userfaultfd.c                         |  15 +-
 include/linux/userfaultfd_k.h            |   1 +
 mm/mremap.c                              | 502 ++++++++++++++---------
 tools/testing/selftests/mm/mremap_test.c | 145 ++++++-
 4 files changed, 462 insertions(+), 201 deletions(-)

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2.50.0