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Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date:   Wed, 11 Jan 2023 10:54:21 +0100
From:   Ingo Molnar <mingo@kernel.org>
To:     Michal Hocko <mhocko@suse.com>
Cc:     Suren Baghdasaryan <surenb@google.com>, akpm@linux-foundation.org,
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Subject: Re: [PATCH 08/41] mm: introduce CONFIG_PER_VMA_LOCK
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* Michal Hocko <mhocko@suse.com> wrote:

> On Tue 10-01-23 16:44:42, Suren Baghdasaryan wrote:
> > On Tue, Jan 10, 2023 at 4:39 PM Davidlohr Bueso <dave@stgolabs.net> wrote:
> > >
> > > On Mon, 09 Jan 2023, Suren Baghdasaryan wrote:
> > >
> > > >This configuration variable will be used to build the support for VMA
> > > >locking during page fault handling.
> > > >
> > > >This is enabled by default on supported architectures with SMP and MMU
> > > >set.
> > > >
> > > >The architecture support is needed since the page fault handler is called
> > > >from the architecture's page faulting code which needs modifications to
> > > >handle faults under VMA lock.
> > >
> > > I don't think that per-vma locking should be something that is user-configurable.
> > > It should just be depdendant on the arch. So maybe just remove CONFIG_PER_VMA_LOCK?
> > 
> > Thanks for the suggestion! I would be happy to make that change if
> > there are no objections. I think the only pushback might have been the
> > vma size increase but with the latest optimization in the last patch
> > maybe that's less of an issue?
> 
> Has vma size ever been a real problem? Sure there might be a lot of those 
> but your patch increases it by rwsem (without the last patch) which is 
> something like 40B on top of 136B vma so we are talking about 400B in 
> total which even with wild mapcount limits shouldn't really be 
> prohibitive. With a default map count limit we are talking about 2M 
> increase at most (per address space).
> 
> Or are you aware of any specific usecases where vma size is a real 
> problem?

40 bytes for the rwsem, plus the patch also adds a 32-bit sequence counter:

  + int vm_lock_seq;
  + struct rw_semaphore lock;

So it's +44 bytes.

Thanks,

	Ingo