[v3 PATCH 0/8] Make khugepaged collapse readonly FS THP more consistent

[Yang Shi]

Changelog
v3: * Register mm to khugepaged in common mmap path instead of touching
      filesystem code (patch 8/8) suggested by Ted.
    * New patch 7/8 cleaned up and renamed khugepaged_enter_vma_merge()
      to khugepaged_enter_vma().
    * Collected acked-by from Song Liu for patch 1 ~ 6.
    * Rebased on top of 5.18-rc1.
    * Excluded linux-xfs and linux-ext4 list since the series doesn't
      touch fs code anymore, but keep linux-fsdevel posted. 
v2: * Collected reviewed-by tags from Miaohe Lin.
    * Fixed build error for patch 4/8.

The readonly FS THP relies on khugepaged to collapse THP for suitable
vmas.  But it is kind of "random luck" for khugepaged to see the
readonly FS vmas (see report: https://lore.kernel.org/linux-mm/00f195d4-d039-3cf2-d3a1-a2c88de397a0@xxxxxxx/) since currently the vmas are registered to khugepaged when:
  - Anon huge pmd page fault
  - VMA merge
  - MADV_HUGEPAGE
  - Shmem mmap

If the above conditions are not met, even though khugepaged is enabled
it won't see readonly FS vmas at all.  MADV_HUGEPAGE could be specified
explicitly to tell khugepaged to collapse this area, but when khugepaged
mode is "always" it should scan suitable vmas as long as VM_NOHUGEPAGE
is not set.

So make sure readonly FS vmas are registered to khugepaged to make the
behavior more consistent.

Registering suitable vmas in common mmap path, that could cover both
readonly FS vmas and shmem vmas, so removed the khugepaged calls in
shmem.c.

The patch 1 ~ 7 are minor bug fixes, clean up and preparation patches.
The patch 8 is the real meat. 


Tested with khugepaged test in selftests and the testcase provided by
Vlastimil Babka in https://lore.kernel.org/lkml/df3b5d1c-a36b-2c73-3e27-99e74983de3a@xxxxxxx/
by commenting out MADV_HUGEPAGE call.


Yang Shi (8):
      sched: coredump.h: clarify the use of MMF_VM_HUGEPAGE
      mm: khugepaged: remove redundant check for VM_NO_KHUGEPAGED
      mm: khugepaged: skip DAX vma
      mm: thp: only regular file could be THP eligible
      mm: khugepaged: make khugepaged_enter() void function
      mm: khugepaged: move some khugepaged_* functions to khugepaged.c
      mm: khugepaged: introduce khugepaged_enter_vma() helper
      mm: mmap: register suitable readonly file vmas for khugepaged

 include/linux/huge_mm.h        | 14 ++++++++++++
 include/linux/khugepaged.h     | 59 ++++++++++++---------------------------------------
 include/linux/sched/coredump.h |  3 ++-
 kernel/fork.c                  |  4 +---
 mm/huge_memory.c               | 15 ++++---------
 mm/khugepaged.c                | 76 +++++++++++++++++++++++++++++++++++++-----------------------------
 mm/mmap.c                      | 14 ++++++++----
 mm/shmem.c                     | 12 -----------
 8 files changed, 88 insertions(+), 109 deletions(-)