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(-)
-- 2.50.0