[PATCH 08/41] mm: introduce CONFIG_PER_VMA_LOCK

[David Laight]

From: Ingo Molnar
> Sent: 11 January 2023 09:54
> 
> * Michal Hocko <mhocko at 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 at 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.

Depend in whether vm_lock_seq goes into a padding hole or not
it will be 40 or 48 bytes.

But if these structures are allocated individually (not an array)
then it depends on how may items kmalloc() fits into a page (or 2,4).

	David

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