[RFC PATCH v4 0/3] mm: retry page faults under per-VMA lock when possible

This v4 reworks the v3 approach in response to review feedback: it keeps
VM_FAULT_RETRY unchanged and introduces VM_FAULT_MAY_USE_VMA_LOCK as an
advisory opt-in bit. Fault handlers can return
VM_FAULT_RETRY | VM_FAULT_MAY_USE_VMA_LOCK when a retry may continue
under the per-VMA lock, and architecture fault handlers perform at most
one bounded VMA-lock retry by setting FAULT_FLAG_TRIED before retrying.
This preserves the existing fallback behavior for retry paths that do
not opt in.

Patch 1 adds the new advisory bit and the bounded retry plumbing.
Patch 2 lets filemap faults opt in to VMA-lock retry.
Patch 3 lets swap faults opt in to VMA-lock retry.

We tested this on a 20-core Intel i7-12700 desktop with a 2GB swapfile.
For the filemap workload, we adapted the v2 stress model to a 20-core
desktop by reducing the thread count and adjusting the memcg limits. The
benchmark uses concurrent file-backed page faults under memcg pressure
with parallel munmap to amplify mmap_lock read-write contention.
Throughput improved by 31.3% to 69.5%, while mmap_lock contention
dropped by about 97.8% to 97.9%.

For the swap workload, we use a non-zero anonymous mapping under memcg
pressure with optional mmap writer threads. Throughput improved by 6.3%
to 47.1%, with the largest gain under four mmap writers.

The detailed benchmark tables are in the individual patch changelogs.
Build testing was done with C=1 on x86, arm, arm64, loongarch, powerpc,
riscv and s390, with no new warnings in the touched files.

Changes since v3:
- Keep VM_FAULT_RETRY unchanged and add VM_FAULT_MAY_USE_VMA_LOCK as an advisory bit
- Bound VMA-lock retries with FAULT_FLAG_TRIED
- Opt in filemap_fault() and do_swap_page() to VM_FAULT_MAY_USE_VMA_LOCK
- Rebased on mm-unstable

Link to v3:
https://lore.kernel.org/all/[email protected]/

Link to v2:
https://lore.kernel.org/all/[email protected]/

Link to v1:
https://lore.kernel.org/all/[email protected]/

Hongru Zhang (3):
  mm: allow page faults to request VMA-lock retry
  mm/filemap: allow filemap faults to retry under the VMA lock
  mm/swap: allow swap faults to retry under the VMA lock

 arch/arm/mm/fault.c       |  6 ++++--
 arch/arm64/mm/fault.c     |  7 +++++--
 arch/loongarch/mm/fault.c |  6 ++++--
 arch/powerpc/mm/fault.c   |  6 ++++--
 arch/riscv/mm/fault.c     |  6 ++++--
 arch/s390/mm/fault.c      |  5 +++--
 arch/x86/mm/fault.c       |  6 ++++--
 include/linux/mm.h        | 28 ++++++++++++++++++++++++++++
 include/linux/mm_types.h  |  4 ++++
 mm/filemap.c              |  2 +-
 mm/memory.c               |  4 +++-
 11 files changed, 64 insertions(+), 16 deletions(-)


base-commit: 486855239e38a91d3eae26618ccb7845c1764625
-- 
2.43.0