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/20230227173632.3292573-34-surenb@xxxxxxxxxx/ [2] https://lore.kernel.org/all/ZsQyI%2F087V34JoIt@xsang-OptiPlex-9020/ [3] https://lore.kernel.org/all/CAJuCfpEisU8Lfe96AYJDZ+OM4NoPmnw9bP53cT_kbfP_pR+-2g@xxxxxxxxxxxxxx/ [4] https://lore.kernel.org/all/20241117080931.600731-1-surenb@xxxxxxxxxx/ 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 -- 2.47.0.338.g60cca15819-goog
