linux-kernel - Re: [PATCH 39/41] kernel/fork: throttle call_rcu() calls in vm_area

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Message-ID: <CAJuCfpEz_Qm3BeY5a0O+OBgZTTLSgvkqDE4XieCYyGMMv3Hykw@mail.gmail.com>
Date:   Mon, 23 Jan 2023 12:08:21 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        akpm@...ux-foundation.org, michel@...pinasse.org,
        jglisse@...gle.com, vbabka@...e.cz, hannes@...xchg.org,
        mgorman@...hsingularity.net, dave@...olabs.net,
        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,
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        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 39/41] kernel/fork: throttle call_rcu() calls in vm_area_free

On Mon, Jan 23, 2023 at 12:00 PM Michal Hocko <mhocko@...e.com> wrote:
>
> On Mon 23-01-23 19:30:43, Matthew Wilcox wrote:
> > On Mon, Jan 23, 2023 at 08:18:37PM +0100, Michal Hocko wrote:
> > > On Mon 23-01-23 18:23:08, Matthew Wilcox wrote:
> > > > On Mon, Jan 23, 2023 at 09:46:20AM -0800, Suren Baghdasaryan wrote:
> > > [...]
> > > > > Yes, batching the vmas into a list and draining it in remove_mt() and
> > > > > exit_mmap() as you suggested makes sense to me and is quite simple.
> > > > > Let's do that if nobody has objections.
> > > >
> > > > I object.  We *know* nobody has a reference to any of the VMAs because
> > > > you have to have a refcount on the mm before you can get a reference
> > > > to a VMA.  If Michal is saying that somebody could do:
> > > >
> > > >   mmget(mm);
> > > >   vma = find_vma(mm);
> > > >   lock_vma(vma);
> > > >   mmput(mm);
> > > >   vma->a = b;
> > > >   unlock_vma(mm, vma);
> > > >
> > > > then that's something we'd catch in review -- you obviously can't use
> > > > the mm after you've dropped your reference to it.
> > >
> > > I am not claiming this is possible now. I do not think we want to have
> > > something like that in the future either but that is really hard to
> > > envision. I am claiming that it is subtle and potentially error prone to
> > > have two different ways of mass vma freeing wrt. locking. Also, don't we
> > > have a very similar situation during last munmaps?
> >
> > We shouldn't have two ways of mass VMA freeing.  Nobody's suggesting that.
> > There are two cases; there's munmap(), which typically frees a single
> > VMA (yes, theoretically, you can free hundreds of VMAs with a single
> > call which spans multiple VMAs, but in practice that doesn't happen),
> > and there's exit_mmap() which happens on exec() and exit().
>
> This requires special casing remove_vma for those two different paths
> (exit_mmap and remove_mt).  If you ask me that sounds like a suboptimal
> code to even not handle potential large munmap which might very well be
> a rare thing as you say. But haven't we learned that sooner or later we
> will find out there is somebody that cares afterall? Anyway, this is not
> something I care about all that much. It is just weird to special case
> exit_mmap, if you ask me. Up to Suren to decide which way he wants to
> go. I just really didn't like the initial implementation of batching
> based on a completely arbitrary batch limit and lazy freeing.

I would prefer to go with the simplest sufficient solution. A
potential issue with a large munmap might prove to be real but I think
we know how to easily fix that with batching if the issue ever
materializes (I'll have a fix ready implementing Michal's suggestion).
So, I suggest going with Liam's/Matthew's solution and converting to
Michal's solution if regression shows up anywhere else. Would that be
acceptable?

> --
> Michal Hocko
> SUSE Labs