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

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Message-ID: <20241219184642.GF26279@noisy.programming.kicks-ass.net>
Date: Thu, 19 Dec 2024 19:46:42 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
	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 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?