linux-kernel - Re: [PATCH v8 00/16] move per-vma lock into vm_area

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Message-ID: <CAJuCfpFS3M50UGi+zdxoe3Lziv_3vbsSwpjWK=WiCXGqxgE1UQ@mail.gmail.com>
Date: Thu, 9 Jan 2025 07:48:32 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: akpm@...ux-foundation.org, willy@...radead.org, liam.howlett@...cle.com, 
	lorenzo.stoakes@...cle.com, mhocko@...e.com, vbabka@...e.cz, 
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Subject: Re: [PATCH v8 00/16] move per-vma lock into vm_area_struct

On Thu, Jan 9, 2025 at 3:51 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> 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 :-(

Oh, no. I think it does. That's why in [1] I introduce
vma_iter_store_attached() to be used on already attached vmas and to
avoid marking them attached again. Also I added assertions in
vma_mark_attached()/vma_mark_detached() to avoid re-attaching or
re-detaching. Unless I misunderstood your comment?

[1] https://lore.kernel.org/all/20250109023025.2242447-5-surenb@google.com/