Subject: [PATCH v3 00/16] mm: expand mmap_prepare functionality and usage

From: "Lorenzo Stoakes (Oracle)" <[email-masked]>
Date: Thu, 19 Mar 2026 18:23:24 +0000

This series expands the mmap_prepare functionality, which is intended to
replace the deprecated f_op->mmap hook which has been the source of bugs
and security issues for some time.

This series starts with some cleanup of existing mmap_prepare logic, then
adds documentation for the mmap_prepare call to make it easier for
filesystem and driver writers to understand how it works.

It then importantly adds a vm_ops->mapped hook, a key feature that was
missing from mmap_prepare previously - this is invoked when a driver which
specifies mmap_prepare has successfully been mapped but not merged with
another VMA.

mmap_prepare is invoked prior to a merge being attempted, so you cannot
manipulate state such as reference counts as if it were a new mapping.

The vm_ops->mapped hook allows a driver to perform tasks required at this
stage, and provides symmetry against subsequent vm_ops->open,close calls.

The series uses this to correct the afs implementation which wrongly
manipulated reference count at mmap_prepare time.

It then adds an mmap_prepare equivalent of vm_iomap_memory() -
mmap_action_simple_ioremap(), then uses this to update a number of drivers.

It then splits out the mmap_prepare compatibility layer (which allows for
invocation of mmap_prepare hooks in an mmap() hook) in such a way as to
allow for more incremental implementation of mmap_prepare hooks.

It then uses this to extend mmap_prepare usage in drivers.

Finally it adds an mmap_prepare equivalent of vm_map_pages(), which lays
the foundation for future work which will extend mmap_prepare to DMA
coherent mappings.

v3:
* Propagated tags (thanks Suren, Richard!)
* Updated 12/16 to correctly clear the vm_area_desc data structure in
  set_desc_from_vma() as per Joshua Hahn (thanks! :)
* Fixed type in 12/16 as per Suren (cheers!)
* Fixed up 6/16 to use mmap_action_ioremap_full() in simple_ioremap_prepare() as
  suggested by Suren.
* Also fixed up 6/16 to call io_remap_pfn_range_prepare() direct rather than
  mmap_action_prepare() as per Suren.
* Also fixed up 6/16 to pass vm_len rather than vm_[start, end] to
  __simple_ioremap_prep() as per Suren (thanks for all the above! :)
* Fixed issue in rmap lock being held - we were referencing a vma->vm_file after
  the VMA was unmapped, so UAF. Avoid that. Also do_munmap() relies on rmap lock
  NOT being held or may deadlock, so extend functionality to ensure we drop it
  when it is held on error paths.
* Updated 'area' -> 'vma' variable in 3/16 in VMA test dup.h.
* Fixed up reference to __compat_vma_mmap() in 12/16 commit message.
* Updated 1/16 to no longer duplicatively apply io_remap_pfn_range_pfn().
* Updated 1/16 to delegate I/O remap complete to remap complete logic.
* Fixed various typos in 12/16.
* Fixed stale comment typos in 13/16.
* Fixed commit msg and comment typos in 14/16.
* Removed accidental sneak peak to future functionality in 15/16 commit message
  :).
* Fixed up field names to be identical in VMA tests + mm_types.h in 6/16,
  15/16.

v2:
* Rebased on
  https://lore.kernel.org/all/[email-masked]/ to make
  Andrew's life easier :)
* Folded all interim fixes into series (thanks Randy for many doc fixes!))
* As per Suren, removed a comment about allocations too small to fail.
* As per Randy, fixed up typo in documentation for vm_area_desc.
* Fixed mmap_action_prepare() not returning if invalid action->type
  specified, as updated from Andrew's interim fix (thanks!) and also
  reported by kernel test bot.
* Updated mmap_action_prepare() and specific prepare functions to only
  pass vm_area_desc parameter as per Suren.
* Fixed up whitespace as per Suren.
* Updated vm_op->open comment in vm_operations_struct to reference forking
  as per Suren.
* Added a commit to check that input range is within VMA on remap as per
  Suren (this also covers I/O remap and all other cases already asserted).
* Updated AFS to not incorrectly reference count on mmap prepare as per
  Usama.
* Also updated various static AFS functions to be consistent with each
  other.
* Updated AFS commit message to reflect mmap_prepare being before any VMA
  merging as per Suren.
* Updated __compat_vma_mapped() to check for NULL vm_ops as per Usama.
* Updated __compat_vma_mapped() to not reference an unmapped VMA's fields
  as per Usama.
* Updated __vma_check_mmap_hook() to check for NULL vm_ops as per Usama.
* Dropped comment about preferring mmap_prepare as seems overly confusing,
  as per Suren.
* Updated the mmap lock assert in unmap_vma_locked() to a write lock assert
  as per Suren.
* Copied vm_ops->open comment over to VMA tests in appropriate patch as per
  Suren.
* Updated mmap_prepare documentation to reflect the fact that no resources
  should be allocated upon mmap_prepare.
* Updated mmap_prepare documentation to reference the vm_ops->mapped
  callback.
* Fixed stray markdown '## How to use' in documentation.
* Fixed bug reported by kernel test bot re: overlooked
  vma_desc_test_flags() -> vma_desc_test() in MTD driver for nommu.
https://lore.kernel.org/linux-mm/[email-masked]/

v1:
https://lore.kernel.org/linux-mm/[email-masked]/

Lorenzo Stoakes (Oracle) (16):
  mm: various small mmap_prepare cleanups
  mm: add documentation for the mmap_prepare file operation callback
  mm: document vm_operations_struct->open the same as close()
  mm: add vm_ops->mapped hook
  fs: afs: correctly drop reference count on mapping failure
  mm: add mmap_action_simple_ioremap()
  misc: open-dice: replace deprecated mmap hook with mmap_prepare
  hpet: replace deprecated mmap hook with mmap_prepare
  mtdchar: replace deprecated mmap hook with mmap_prepare, clean up
  stm: replace deprecated mmap hook with mmap_prepare
  staging: vme_user: replace deprecated mmap hook with mmap_prepare
  mm: allow handling of stacked mmap_prepare hooks in more drivers
  drivers: hv: vmbus: replace deprecated mmap hook with mmap_prepare
  uio: replace deprecated mmap hook with mmap_prepare in uio_info
  mm: add mmap_action_map_kernel_pages[_full]()
  mm: on remap assert that input range within the proposed VMA

 Documentation/driver-api/vme.rst           |   2 +-
 Documentation/filesystems/index.rst        |   1 +
 Documentation/filesystems/mmap_prepare.rst | 168 ++++++++++++++
 drivers/char/hpet.c                        |  12 +-
 drivers/hv/hyperv_vmbus.h                  |   4 +-
 drivers/hv/vmbus_drv.c                     |  31 ++-
 drivers/hwtracing/stm/core.c               |  31 ++-
 drivers/misc/open-dice.c                   |  19 +-
 drivers/mtd/mtdchar.c                      |  21 +-
 drivers/staging/vme_user/vme.c             |  20 +-
 drivers/staging/vme_user/vme.h             |   2 +-
 drivers/staging/vme_user/vme_user.c        |  51 +++--
 drivers/target/target_core_user.c          |  26 ++-
 drivers/uio/uio.c                          |  10 +-
 drivers/uio/uio_hv_generic.c               |  11 +-
 fs/afs/file.c                              |  36 ++-
 include/linux/fs.h                         |  14 +-
 include/linux/hyperv.h                     |   4 +-
 include/linux/mm.h                         | 159 ++++++++++++-
 include/linux/mm_types.h                   |  17 +-
 include/linux/uio_driver.h                 |   4 +-
 mm/internal.h                              |  41 ++--
 mm/memory.c                                | 175 ++++++++++----
 mm/util.c                                  | 251 ++++++++++++++-------
 mm/vma.c                                   |  53 +++--
 mm/vma.h                                   |   2 +-
 tools/testing/vma/include/dup.h            | 152 ++++++++++---
 tools/testing/vma/include/stubs.h          |   9 +-
 28 files changed, 990 insertions(+), 336 deletions(-)
 create mode 100644 Documentation/filesystems/mmap_prepare.rst

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