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

[Lorenzo Stoakes]

+cc linux-api, FYI - apologies I intended to cc from the start, was simply
an oversight. All future respins will cc.

This series changes mremap() semantics (I will update the manpage
accordingly of course).

Cheers, Lorenzo

On Mon, Jul 07, 2025 at 06:27:43AM +0100, Lorenzo Stoakes wrote:
> 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