[PATCH 0/4] Abstract vma_merge() and split_vma()

[Lorenzo Stoakes]

The vma_merge() interface is very confusing and its implementation has led
to numerous bugs as a result of that confusion.

In addition there is duplication both in invocation of vma_merge(), but
also in the common mprotect()-style pattern of attempting a merge, then if
this fails, splitting the portion of a VMA about to have its attributes
changed.

This pattern has been copy/pasted around the kernel in each instance where
such an operation has been required, each very slightly modified from the
last to make it even harder to decipher what is going on.

Simplify the whole thing by dividing the actual uses of vma_merge() and
split_vma() into specific and abstracted functions and de-duplicate the
vma_merge()/split_vma() pattern altogether.

Doing so also opens the door to changing how vma_merge() is implemented -
by knowing precisely what cases a caller is invoking rather than having a
central interface where anything might happen, we can untangle the brittle
and confusing vma_merge() implementation into something more workable.

For mprotect()-like cases we introduce vma_modify() which performs the
vma_merge()/split_vma() pattern, returning a pointer or an ERR_PTR(err) if
the splits fail.

This is an internal interface, as it is confusing having a number of
different parameters available for fields that can be changed. Instead we
split the kernel interface into four functions:-

* vma_modify_flags()      - Prepare to modify the VMA's flags.
* vma_modify_flags_name() - Prepare to modify the VMA's flags/anon_vma_name
* vma_modify_policy()     - Prepare to modify the VMA's mempolicy.
* vma_modify_uffd()       - Prepare to modify the VMA's flags/uffd context.

For cases where a new VMA is attempted to be merged with adjacent VMAs we
add:-

* vma_merge_new_vma() - Prepare to merge a new VMA.
* vma_merge_extend()  - Prepare to extend the end of a new VMA.

Lorenzo Stoakes (4):
  mm: abstract the vma_merge()/split_vma() pattern for mprotect() et al.
  mm: make vma_merge() and split_vma() internal
  mm: abstract merge for new VMAs into vma_merge_new_vma()
  mm: abstract VMA extension and merge into vma_merge_extend() helper

 fs/userfaultfd.c   |  53 +++++----------
 include/linux/mm.h |  32 ++++++---
 mm/internal.h      |   7 ++
 mm/madvise.c       |  25 ++-----
 mm/mempolicy.c     |  20 +-----
 mm/mlock.c         |  24 ++-----
 mm/mmap.c          | 160 ++++++++++++++++++++++++++++++++++++++++-----
 mm/mprotect.c      |  27 ++------
 mm/mremap.c        |  30 ++++-----
 mm/nommu.c         |   4 +-
 10 files changed, 228 insertions(+), 154 deletions(-)

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2.42.0