[v4,0/5] move per-vma lock into vm_area_struct

Message

Suren Baghdasaryan Nov. 20, 2024, 12:08 a.m. UTC

Back when per-vma locks were introduces, vm_lock was moved out of
vm_area_struct in [1] because of the performance regression caused by
false cacheline sharing. Recent investigation [2] revealed that the
regressions is limited to a rather old Broadwell microarchitecture and
even there it can be mitigated by disabling adjacent cacheline
prefetching, see [3].
Splitting single logical structure into multiple ones leads to more
complicated management, extra pointer dereferences and overall less
maintainable code. When that split-away part is a lock, it complicates
things even further. With no performance benefits, there are no reasons
for this split. Merging the vm_lock back into vm_area_struct also allows
vm_area_struct to use SLAB_TYPESAFE_BY_RCU later in this patchset.
This patchset:
1. moves vm_lock back into vm_area_struct, aligning it at the cacheline
boundary and changing the cache to be cacheline-aligned to minimize
cacheline sharing;
2. changes vm_area_struct initialization to mark new vma as detached until
it is inserted into vma tree;
3. changes vm_area_struct cache to SLAB_TYPESAFE_BY_RCU to allow for their
reuse and to minimize call_rcu() calls.
Pagefault microbenchmarks do not show noticeable performance change.

Changes since v3 [4]
- Added SOBs, per Lorenzo Stoakes and Davidlohr Bueso;
- Replaced vma write-locking in vma_mark_attached() with memory barriers to
order accesses to vma->detached vs vm_mm/vm_start/vm_end

Patch applies over mm-unstable

[1] https://lore.kernel.org/all/[e-mail]/
[2] https://lore.kernel.org/all/ZsQyI%2F087V34JoIt@xsang-OptiPlex-9020/
[3] https://lore.kernel.org/all/[e-mail]/
[4] https://lore.kernel.org/all/[e-mail]/

Suren Baghdasaryan (5):
  mm: introduce vma_start_read_locked{_nested} helpers
  mm: move per-vma lock into vm_area_struct
  mm: mark vma as detached until it's added into vma tree
  mm: make vma cache SLAB_TYPESAFE_BY_RCU
  docs/mm: document latest changes to vm_lock

 Documentation/mm/process_addrs.rst |  10 ++-
 include/linux/mm.h                 | 129 ++++++++++++++++++++++++-----
 include/linux/mm_types.h           |  19 ++---
 kernel/fork.c                      |  88 +++++++-------------
 mm/memory.c                        |  17 ++--
 mm/userfaultfd.c                   |  22 ++---
 mm/vma.c                           |   8 +-
 mm/vma.h                           |   2 +
 tools/testing/vma/vma_internal.h   |  55 +++++-------
 9 files changed, 198 insertions(+), 152 deletions(-)


base-commit: 5a7056135bb69da2ce0a42eb8c07968c1331777b