Hardening page _refcount

Changelog:
v3:
- Sync with the latest linux-next
v2:
- As suggested by Matthew Wilcox removed "mm: page_ref_add_unless()
  does not trace 'u' argument" patch as page_ref_add_unless is going
  away.
v1:
- sync with the latest linux-next
  RFCv2:
- use the "fetch" variant instead of "return" of atomic instructions
- allow negative values, as we are using all 32-bits of _refcount.


It is hard to root cause _refcount problems, because they usually
manifest after the damage has occurred.  Yet, they can lead to
catastrophic failures such memory corruptions. There were a number
of refcount related issues discovered recently [1], [2], [3].

Improve debugability by adding more checks that ensure that
page->_refcount never turns negative (i.e. double free does not
happen, or free after freeze etc).

- Check for overflow and underflow right from the functions that
  modify _refcount
- Remove set_page_count(), so we do not unconditionally overwrite
  _refcount with an unrestrained value
- Trace return values in all functions that modify _refcount

Applies against next-20220125.

Previous verions:
v2: https://lore.kernel.org/all/20211221150140.988298-1-pasha...
v1: https://lore.kernel.org/all/20211208203544.2297121-1-pash...
RFCv2: https://lore.kernel.org/all/20211117012059.141450-1-pasha...
RFCv1: https://lore.kernel.org/all/20211026173822.502506-1-pasha...

[1] https://lore.kernel.org/all/xr9335nxwc5y.fsf@gthelen2.svl...
[2] https://lore.kernel.org/all/1582661774-30925-2-git-send-e...
[3] https://lore.kernel.org/all/20210622021423.154662-3-mike....

Pasha Tatashin (9):
  mm: add overflow and underflow checks for page->_refcount
  mm: Avoid using set_page_count() in set_page_recounted()
  mm: remove set_page_count() from page_frag_alloc_align
  mm: avoid using set_page_count() when pages are freed into allocator
  mm: rename init_page_count() -> page_ref_init()
  mm: remove set_page_count()
  mm: simplify page_ref_* functions
  mm: do not use atomic_set_release in page_ref_unfreeze()
  mm: use atomic_cmpxchg_acquire in page_ref_freeze().

 arch/m68k/mm/motorola.c         |   2 +-
 include/linux/mm.h              |   2 +-
 include/linux/page_ref.h        | 149 +++++++++++++++-----------------
 include/trace/events/page_ref.h |  58 ++++++++-----
 mm/debug_page_ref.c             |  22 +----
 mm/internal.h                   |   6 +-
 mm/page_alloc.c                 |  19 ++--
 7 files changed, 132 insertions(+), 126 deletions(-)

-- 
2.35.0.rc0.227.g00780c9af4-goog