Re: [PATCH v3 00/10] mm/mremap: permit mremap() move of multiple VMAs

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

Apologies, I've done it again... :) Friday eh?

+cc linux-api - FYI guys.

I will update manpage to reflect this change of course also.

On Fri, Jul 11, 2025 at 12:38:14PM +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.
>
> It is also not compatible with file-backed mappings with customised
> get_unmapped_area() handlers as these may not honour MREMAP_FIXED.
>
> The input and output addresses ranges must not overlap. We carefully
> account for moves which would result in VMA iterator invalidation.
>
> While there can be gaps between VMAs in the input range, there can be no
> gap before the first VMA in the range.
>
>
> v3:
> * Disallowed move operation except for MREMAP_FIXED.
> * Disallow gap at start of aggregate range to avoid confusion.
> * Disallow any file-baked VMAs with custom get_unmapped_area.
> * Renamed multi_vma to seen_vma to be clearer. Stop reusing new_addr, use
>   separate target_addr var to track next target address.
> * Check if first VMA fails multi VMA check, if so we'll allow one VMA but
>   not multiple.
> * Updated the commit message for patch 9 to be clearer about gap behaviour.
> * Removed accidentally included debug goto statement in test (doh!). Test
>   was and is passing regardless.
> * Unmap target range in test, previously we ended up moving additional VMAs
>   unintentionally. This still all passed :) but was not what was intended.
> * Removed self-merge check - there is absolutely no way this can happen
>   across multiple VMAs, as there is no means of moving VMAs such that a VMA
>   merges with itself.
>
> v2:
> * Squashed uffd stub fix into series.
> * Propagated tags, thanks!
> * Fixed param naming in patch 4 as per Vlastimil.
> * Renamed vma_reset to vmi_needs_reset + dropped reset on unmap as per
>   Liam.
> * Correctly return -EFAULT if no VMAs in input range.
> * Account for get_unmapped_area() disregarding MAP_FIXED and returning an
>   altered address.
> * Added additional explanatatory comment to the remap_move() function.
> https://lore.kernel.org/all/cover.1751865330.git.lorenzo.stoakes@oracle.com/
>
> v1:
> https://lore.kernel.org/all/cover.1751865330.git.lorenzo.stoakes@oracle.com/
>
>
> 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            |   5 +
>  mm/mremap.c                              | 553 +++++++++++++++--------
>  tools/testing/selftests/mm/mremap_test.c | 146 +++++-
>  4 files changed, 518 insertions(+), 201 deletions(-)
>
> --
> 2.50.0