[PATCH v2 00/14] introduce pte_offset_map_{ro|rw}_nolock()

[Qi Zheng <zhengqi.arch@bytedance.com>]

Changes in v2:
 - rename pte_offset_map_{readonly|maywrite}_nolock() to
   pte_offset_map_{ro|rw}_nolock() (LEROY Christophe)
 - make pte_offset_map_rw_nolock() not accept NULL parameters
   (David Hildenbrand)
 - rebase onto the next-20240822

Hi all,

As proposed by David Hildenbrand [1], this series introduces the following two
new helper functions to replace pte_offset_map_nolock().

1. pte_offset_map_ro_nolock()
2. pte_offset_map_rw_nolock()

As the name suggests, pte_offset_map_ro_nolock() is used for read-only
case. In this case, only read-only operations will be performed on PTE page
after the PTL is held. The RCU lock in pte_offset_map_nolock() will ensure that
the PTE page will not be freed, and there is no need to worry about whether the
pmd entry is modified. Therefore pte_offset_map_ro_nolock() is just a renamed
version of pte_offset_map_nolock().

pte_offset_map_rw_nolock() is used for may-write case. In this case, the pte or
pmd entry may be modified after the PTL is held, so we need to ensure that the
pmd entry has not been modified concurrently. So in addition to the name change,
it also outputs the pmdval when successful. This can help the caller recheck
*pmd once the PTL is taken. In some cases, that is, either the mmap_lock for
write, or pte_same() check on contents, is also enough to ensure that the pmd
entry is stable. But in order to prevent the interface from being abused, we
choose to pass in a dummy local variable instead of NULL.

This series will convert all pte_offset_map_nolock() into the above two helper
functions one by one, and finally completely delete it.

This also a preparation for reclaiming the empty user PTE page table pages.

This series is based on the next-20240822.

Comments and suggestions are welcome!

Thanks,
Qi

[1]. https://lore.kernel.org/lkml/f79bbfc9-bb4c-4da4-9902-2e73817dd135@redhat.com/

Qi Zheng (14):
  mm: pgtable: introduce pte_offset_map_{ro|rw}_nolock()
  arm: adjust_pte() use pte_offset_map_rw_nolock()
  powerpc: assert_pte_locked() use pte_offset_map_ro_nolock()
  mm: filemap: filemap_fault_recheck_pte_none() use
    pte_offset_map_ro_nolock()
  mm: khugepaged: __collapse_huge_page_swapin() use
    pte_offset_map_ro_nolock()
  mm: handle_pte_fault() use pte_offset_map_rw_nolock()
  mm: khugepaged: collapse_pte_mapped_thp() use
    pte_offset_map_rw_nolock()
  mm: copy_pte_range() use pte_offset_map_rw_nolock()
  mm: mremap: move_ptes() use pte_offset_map_rw_nolock()
  mm: page_vma_mapped_walk: map_pte() use pte_offset_map_rw_nolock()
  mm: userfaultfd: move_pages_pte() use pte_offset_map_rw_nolock()
  mm: multi-gen LRU: walk_pte_range() use pte_offset_map_rw_nolock()
  mm: pgtable: remove pte_offset_map_nolock()
  mm: khugepaged: retract_page_tables() use pte_offset_map_rw_nolock()

 Documentation/mm/split_page_table_lock.rst |  6 +++-
 arch/arm/mm/fault-armv.c                   |  9 ++++-
 arch/powerpc/mm/pgtable.c                  |  2 +-
 include/linux/mm.h                         |  7 ++--
 mm/filemap.c                               |  4 +--
 mm/khugepaged.c                            | 39 ++++++++++++++++++--
 mm/memory.c                                | 23 ++++++++++--
 mm/mremap.c                                |  9 ++++-
 mm/page_vma_mapped.c                       | 24 ++++++++++---
 mm/pgtable-generic.c                       | 42 ++++++++++++++++------
 mm/userfaultfd.c                           | 15 ++++++--
 mm/vmscan.c                                |  9 ++++-
 12 files changed, 157 insertions(+), 32 deletions(-)

-- 
2.20.1