Date:   Mon, 23 Jan 2023 20:18:37 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Suren Baghdasaryan <surenb@google.com>,
        "Liam R. Howlett" <Liam.Howlett@oracle.com>,
        akpm@linux-foundation.org, michel@lespinasse.org,
        jglisse@google.com, vbabka@suse.cz, hannes@cmpxchg.org,
        mgorman@techsingularity.net, dave@stgolabs.net,
        peterz@infradead.org, ldufour@linux.ibm.com,
        laurent.dufour@fr.ibm.com, paulmck@kernel.org, luto@kernel.org,
        songliubraving@fb.com, peterx@redhat.com, david@redhat.com,
        dhowells@redhat.com, hughd@google.com, bigeasy@linutronix.de,
        kent.overstreet@linux.dev, punit.agrawal@bytedance.com,
        lstoakes@gmail.com, peterjung1337@gmail.com, rientjes@google.com,
        axelrasmussen@google.com, joelaf@google.com, minchan@google.com,
        jannh@google.com, shakeelb@google.com, tatashin@google.com,
        edumazet@google.com, gthelen@google.com, gurua@google.com,
        arjunroy@google.com, soheil@google.com, hughlynch@google.com,
        leewalsh@google.com, posk@google.com, linux-mm@kvack.org,
        linux-arm-kernel@lists.infradead.org,
        linuxppc-dev@lists.ozlabs.org, x86@kernel.org,
        linux-kernel@vger.kernel.org, kernel-team@android.com
Subject: Re: [PATCH 39/41] kernel/fork: throttle call_rcu() calls in
 vm_area_free
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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