[v2 RFC PATCH 0/7] mm: thp: use generic THP migration for NUMA hinting fault

[Yang Shi]

Changelog:
v1 --> v2:
    * Adopted the suggestion from Gerald Schaefer to skip huge PMD for S390
      for now.
    * Used PageTransHuge to distinguish base page or THP instead of a new
      parameter for migrate_misplaced_page() per Huang Ying.
    * Restored PMD lazily to avoid unnecessary TLB shootdown per Huang Ying.
    * Skipped shared THP.
    * Updated counters correctly.
    * Rebased to linux-next (next-20210412).

When the THP NUMA fault support was added THP migration was not supported yet.
So the ad hoc THP migration was implemented in NUMA fault handling.  Since v4.14
THP migration has been supported so it doesn't make too much sense to still keep
another THP migration implementation rather than using the generic migration
code.  It is definitely a maintenance burden to keep two THP migration
implementation for different code paths and it is more error prone.  Using the
generic THP migration implementation allows us remove the duplicate code and
some hacks needed by the old ad hoc implementation.

A quick grep shows x86_64, PowerPC (book3s), ARM64 ans S390 support both THP
and NUMA balancing.  The most of them support THP migration except for S390.
Zi Yan tried to add THP migration support for S390 before but it was not
accepted due to the design of S390 PMD.  For the discussion, please see:
https://lkml.org/lkml/2018/4/27/953.

Per the discussion with Gerald Schaefer in v1 it is acceptible to skip huge
PMD for S390 for now.

I saw there were some hacks about gup from git history, but I didn't figure out
if they have been removed or not since I just found FOLL_NUMA code in the current
gup implementation and they seems useful.

I'm trying to keep the behavior as consistent as possible between before and after.
But there is still some minor disparity.  For example, file THP won't
get migrated at all in old implementation due to the anon_vma check, but
the new implementation doesn't need acquire anon_vma lock anymore, so
file THP might get migrated.  Not sure if this behavior needs to be
kept.

Patch #1 ~ #2 are preparation patches.
Patch #3 is the real meat.
Patch #4 ~ #6 keep consistent counters and behaviors with before.
Patch #7 skips change huge PMD to prot_none if thp migration is not supported.

Yang Shi (7):
      mm: memory: add orig_pmd to struct vm_fault
      mm: memory: make numa_migrate_prep() non-static
      mm: thp: refactor NUMA fault handling
      mm: migrate: account THP NUMA migration counters correctly
      mm: migrate: don't split THP for misplaced NUMA page
      mm: migrate: check mapcount for THP instead of ref count
      mm: thp: skip make PMD PROT_NONE if THP migration is not supported

 include/linux/huge_mm.h |   9 ++---
 include/linux/migrate.h |  23 -----------
 include/linux/mm.h      |   3 ++
 mm/huge_memory.c        | 156 +++++++++++++++++++++++++-----------------------------------------------
 mm/internal.h           |  21 ++--------
 mm/memory.c             |  31 +++++++--------
 mm/migrate.c            | 204 +++++++++++++++++++++--------------------------------------------------------------------------
 7 files changed, 123 insertions(+), 324 deletions(-)