Re: [PATCH v8 00/16] move per-vma lock into vm_area_struct

[Peter Zijlstra <peterz@infradead.org>]

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 :-(