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Message-ID: <Y76HTfIeEt8ZOIH3@gmail.com>
Date:   Wed, 11 Jan 2023 10:54:21 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org,
        michel@...pinasse.org, jglisse@...gle.com, vbabka@...e.cz,
        hannes@...xchg.org, mgorman@...hsingularity.net,
        willy@...radead.org, liam.howlett@...cle.com, peterz@...radead.org,
        ldufour@...ux.ibm.com, laurent.dufour@...ibm.com,
        paulmck@...nel.org, luto@...nel.org, songliubraving@...com,
        peterx@...hat.com, david@...hat.com, dhowells@...hat.com,
        hughd@...gle.com, bigeasy@...utronix.de, kent.overstreet@...ux.dev,
        punit.agrawal@...edance.com, lstoakes@...il.com,
        peterjung1337@...il.com, rientjes@...gle.com,
        axelrasmussen@...gle.com, joelaf@...gle.com, minchan@...gle.com,
        jannh@...gle.com, shakeelb@...gle.com, tatashin@...gle.com,
        edumazet@...gle.com, gthelen@...gle.com, gurua@...gle.com,
        arjunroy@...gle.com, soheil@...gle.com, hughlynch@...gle.com,
        leewalsh@...gle.com, posk@...gle.com, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH 08/41] mm: introduce CONFIG_PER_VMA_LOCK


* Michal Hocko <mhocko@...e.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@...olabs.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

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