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. replaces vm_lock and vma->detached flag with a reference counter; 4. changes vm_area_struct cache to SLAB_TYPESAFE_BY_RCU to allow for their reuse and to minimize call_rcu() calls. Pagefault microbenchmarks show performance improvement: Hmean faults/cpu-1 507926.5547 ( 0.00%) 506519.3692 * -0.28%* Hmean faults/cpu-4 479119.7051 ( 0.00%) 481333.6802 * 0.46%* Hmean faults/cpu-7 452880.2961 ( 0.00%) 455845.6211 * 0.65%* Hmean faults/cpu-12 347639.1021 ( 0.00%) 352004.2254 * 1.26%* Hmean faults/cpu-21 200061.2238 ( 0.00%) 229597.0317 * 14.76%* Hmean faults/cpu-30 145251.2001 ( 0.00%) 164202.5067 * 13.05%* Hmean faults/cpu-48 106848.4434 ( 0.00%) 120641.5504 * 12.91%* Hmean faults/cpu-56 92472.3835 ( 0.00%) 103464.7916 * 11.89%* Hmean faults/sec-1 507566.1468 ( 0.00%) 506139.0811 * -0.28%* Hmean faults/sec-4 1880478.2402 ( 0.00%) 1886795.6329 * 0.34%* Hmean faults/sec-7 3106394.3438 ( 0.00%) 3140550.7485 * 1.10%* Hmean faults/sec-12 4061358.4795 ( 0.00%) 4112477.0206 * 1.26%* Hmean faults/sec-21 3988619.1169 ( 0.00%) 4577747.1436 * 14.77%* Hmean faults/sec-30 3909839.5449 ( 0.00%) 4311052.2787 * 10.26%* Hmean faults/sec-48 4761108.4691 ( 0.00%) 5283790.5026 * 10.98%* Hmean faults/sec-56 4885561.4590 ( 0.00%) 5415839.4045 * 10.85%* Changes since v5 [4] - Added Reviewed-by, per Vlastimil Babka; - Added replacement of vm_lock and vma->detached flag with vm_refcnt, per Peter Zijlstra and Matthew Wilcox - Marked vmas detached during exit_mmap; - Ensureed vmas are in detached state before they are freed; - Changed SLAB_TYPESAFE_BY_RCU patch to not require ctor, leading to a much simpler code; - Removed unnecessary patch [5] - Updated documentation to reflect changes to vm_lock; Patchset applies over mm-unstable after reverting v5 of this patchset [4] (currently 687e99a5faa5-905ab222508a) [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/20241206225204.4008261-1-surenb@xxxxxxxxxx/ [5] https://lore.kernel.org/all/20241206225204.4008261-6-surenb@xxxxxxxxxx/ Suren Baghdasaryan (16): 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/nommu: fix the last places where vma is not locked before being attached types: move struct rcuwait into types.h mm: allow vma_start_read_locked/vma_start_read_locked_nested to fail mm: move mmap_init_lock() out of the header file mm: uninline the main body of vma_start_write() refcount: introduce __refcount_{add|inc}_not_zero_limited mm: replace vm_lock and detached flag with a reference count mm: enforce vma to be in detached state before freeing mm: remove extra vma_numab_state_init() call mm: introduce vma_ensure_detached() mm: prepare lock_vma_under_rcu() for vma reuse possibility mm: make vma cache SLAB_TYPESAFE_BY_RCU docs/mm: document latest changes to vm_lock Documentation/mm/process_addrs.rst | 44 ++++---- include/linux/mm.h | 162 +++++++++++++++++++++++------ include/linux/mm_types.h | 37 ++++--- include/linux/mmap_lock.h | 6 -- include/linux/rcuwait.h | 13 +-- include/linux/refcount.h | 20 +++- include/linux/slab.h | 6 -- include/linux/types.h | 12 +++ kernel/fork.c | 88 ++++------------ mm/init-mm.c | 1 + mm/memory.c | 75 +++++++++++-- mm/mmap.c | 8 +- mm/nommu.c | 2 + mm/userfaultfd.c | 31 +++--- mm/vma.c | 15 ++- mm/vma.h | 4 +- tools/testing/vma/linux/atomic.h | 5 + tools/testing/vma/vma_internal.h | 96 +++++++++-------- 18 files changed, 378 insertions(+), 247 deletions(-) -- 2.47.1.613.gc27f4b7a9f-goog
