Re: [PATCH v2 2/5] mm: move per-vma lock into vm_area_struct

[Lorenzo Stoakes]

On Wed, Nov 13, 2024 at 02:28:16PM +0000, Lorenzo Stoakes wrote:
> On Tue, Nov 12, 2024 at 11:46:32AM -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].
>
> I don't see a motivating reason as to why we want to do this? We increase
> memory usage here which is not good, but later lock optimisation mitigates
> it, but why wouldn't we just do the lock optimisations and use less memory
> overall?

I worded this badly. To clarify:

I don't see a motivating reason _in the commit message_ as to why we want
to do this.

I am certain there are, in fact Mateusz and Vlastimil have provided them.

So my review is - let's just put these there :)