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

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Date:   Mon, 23 Jan 2023 20:18:37 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Suren Baghdasaryan <surenb@...gle.com>,
        "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,
        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 39/41] kernel/fork: throttle call_rcu() calls in
 vm_area_free

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?

-- 
Michal Hocko
SUSE Labs