linux-kernel - Re: [PATCH v6 10/16] mm: replace vm_lock and detached flag with a reference count

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Message-ID: <CAJuCfpHBKuwsZMmz_1Lt4ZGuPiiyHeqizrZu4e+mFM+Bf02CGw@mail.gmail.com>
Date: Fri, 20 Dec 2024 07:22:18 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>, Peter Zijlstra <peterz@...radead.org>, 
	Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org, willy@...radead.org, 
	lorenzo.stoakes@...cle.com, mhocko@...e.com, vbabka@...e.cz, 
	hannes@...xchg.org, mjguzik@...il.com, oliver.sang@...el.com, 
	mgorman@...hsingularity.net, david@...hat.com, peterx@...hat.com, 
	oleg@...hat.com, dave@...olabs.net, paulmck@...nel.org, brauner@...nel.org, 
	dhowells@...hat.com, hdanton@...a.com, hughd@...gle.com, 
	lokeshgidra@...gle.com, minchan@...gle.com, jannh@...gle.com, 
	shakeel.butt@...ux.dev, souravpanda@...gle.com, pasha.tatashin@...een.com, 
	klarasmodin@...il.com, corbet@....net, linux-doc@...r.kernel.org, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v6 10/16] mm: replace vm_lock and detached flag with a
 reference count

On Thu, Dec 19, 2024 at 10:55 AM Liam R. Howlett
<Liam.Howlett@...cle.com> wrote:
>
> * Peter Zijlstra <peterz@...radead.org> [241219 13:47]:
> > On Thu, Dec 19, 2024 at 01:18:23PM -0500, Liam R. Howlett wrote:
> >
> > > > For RCU lookups only the mas tree matters -- and its left present there.
> > > >
> > > > If you really want to block RCU readers, I would suggest punching a hole
> > > > in the mm_mt. All the traditional code won't notice anyway, this is all
> > > > with mmap_lock held for writing.
> > >
> > > We don't want to block all rcu readers, we want to block the rcu readers
> > > that would see the problem - that is, anyone trying to read a particular
> > > area.
> > >
> > > Right now we can page fault in unpopulated vmas while writing other vmas
> > > to the tree.  We are also moving more users to rcu reading to use the
> > > vmas they need without waiting on writes to finish.
> > >
> > > Maybe I don't understand your suggestion, but I would think punching a
> > > hole would lose this advantage?
> >
> > My suggestion was to remove the range stuck in mas_detach from mm_mt.
> > That is exactly the affected range, no?
>
> Yes.
>
> But then looping over the vmas will show a gap where there should not be
> a gap.
>
> If we stop rcu readers entirely we lose the advantage.
>
> This is exactly the issue that the locking dance was working around :)

Sorry for not participating in the discussion, folks. I'm down with a
terrible flu and my brain is not working well. I'll catch up once I
get better.

>