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. To avoid bloating vm_area_struct, we introduce unioned freeptr_t field and allow freeptr_offset to be used with ctor in a separate patch. Pagefault microbenchmarks do not show noticeable performance change. Changes since v4 [4] - Added SOBs, per Lorenzo Stoakes and Shakeel Butt; - Changed vma_clear() and vma_copy() to set required vma members individually, per Matthew Wilcox - Added comments in vma_start_read() about new false locked result possibilities, per Vlastimil Babka - Added back freeptr_t into vm_area_struct, as it's harmless and can be used in the future to shrink the structure, per Vlastimil Babka - Fixed the race in vm_area_alloc() when vma->detached was temporarily reset by memcpy() before setting it back, per Vlastimil Babka - Added a patch allowing freeptr_offset to be used with ctor, per Vlastimil Babka Patch applies over linux-next (due to vm_lock change [5] not in mm tree). [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/20241120000826.335387-1-surenb@xxxxxxxxxx/ [5] https://lore.kernel.org/all/20241122174416.1367052-2-surenb@xxxxxxxxxx/ Suren Baghdasaryan (6): 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 mm/slab: allow freeptr_offset to be used with ctor docs/mm: document latest changes to vm_lock Documentation/mm/process_addrs.rst | 10 +- include/linux/mm.h | 107 ++++++++++++++---- include/linux/mm_types.h | 16 ++- include/linux/slab.h | 11 +- kernel/fork.c | 168 ++++++++++++++++++++--------- mm/memory.c | 17 ++- mm/slub.c | 2 +- mm/userfaultfd.c | 22 +--- mm/vma.c | 8 +- mm/vma.h | 2 + tools/testing/vma/vma_internal.h | 55 ++++------ 11 files changed, 267 insertions(+), 151 deletions(-) base-commit: ebe1b11614e079c5e366ce9bd3c8f44ca0fbcc1b -- 2.47.0.338.g60cca15819-goog
