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

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Message-ID: <6nck2rfwcytqdinsavmewytgcca43mldlczmao3zztrpr5v2ci@4xn6nwp6tcih>
Date: Thu, 19 Dec 2024 13:55:17 -0500
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: 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,
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        minchan@...gle.com, jannh@...gle.com, shakeel.butt@...ux.dev,
        souravpanda@...gle.com, pasha.tatashin@...een.com,
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        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

* 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 :)