On Wed, Jan 08, 2025 at 06:30:09PM -0800, Suren Baghdasaryan wrote: > 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. Does not clean up that reattach nonsense :-(
