selftests/mm: add self tests for guard page feature

Utilise the kselftest harmness to implement tests for the guard page
implementation.

We start by implement basic tests asserting that guard pages can be
installed, removed and that touching guard pages result in SIGSEGV.  We
also assert that, in removing guard pages from a range, non-guard pages
remain intact.

We then examine different operations on regions containing guard markers
behave to ensure correct behaviour:

* Operations over multiple VMAs operate as expected.
* Invoking MADV_GUARD_INSTALL / MADV_GUARD_REMOVE via process_madvise() in
  batches works correctly.
* Ensuring that munmap() correctly tears down guard markers.
* Using mprotect() to adjust protection bits does not in any way override
  or cause issues with guard markers.
* Ensuring that splitting and merging VMAs around guard markers causes no
  issue - i.e. that a marker which 'belongs' to one VMA can function just
  as well 'belonging' to another.
* Ensuring that madvise(..., MADV_DONTNEED) and madvise(..., MADV_FREE)
  do not remove guard markers.
* Ensuring that mlock()'ing a range containing guard markers does not
  cause issues.
* Ensuring that mremap() can move a guard range and retain guard markers.
* Ensuring that mremap() can expand a guard range and retain guard
  markers (perhaps moving the range).
* Ensuring that mremap() can shrink a guard range and retain guard markers.
* Ensuring that forking a process correctly retains guard markers.
* Ensuring that forking a VMA with VM_WIPEONFORK set behaves sanely.
* Ensuring that lazyfree simply clears guard markers.
* Ensuring that userfaultfd can co-exist with guard pages.
* Ensuring that madvise(..., MADV_POPULATE_READ) and
  madvise(..., MADV_POPULATE_WRITE) error out when encountering
  guard markers.
* Ensuring that madvise(..., MADV_COLD) and madvise(..., MADV_PAGEOUT) do
  not remove guard markers.

If any test is unable to be run due to lack of permissions, that test is
skipped.

Link: https://lkml.kernel.org/r/c3dcca76b736bac0aeaf1dc085927536a253ac94.1730123433.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <[email protected]>
Reviewed-by: Shuah Khan <[email protected]>
Suggested-by: Vlastimil Babka <[email protected]>
Suggested-by: Jann Horn <[email protected]>
Suggested-by: David Hildenbrand <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Chris Zankel <[email protected]>
Cc: Helge Deller <[email protected]>
Cc: James E.J. Bottomley <[email protected]>
Cc: Jeff Xu <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: Liam R. Howlett <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Matt Turner <[email protected]>
Cc: Max Filippov <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Richard Henderson <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Sidhartha Kumar <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Thomas Bogendoerfer <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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