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

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Message-ID: <20241219085311.GA26551@noisy.programming.kicks-ass.net>
Date: Thu, 19 Dec 2024 09:53:11 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: "Liam R. Howlett" <Liam.Howlett@...cle.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 Wed, Dec 18, 2024 at 09:58:12AM -0800, Suren Baghdasaryan wrote:

> And remove_vma will be just freeing the vmas. Is that correct?

Yep.

> I'm a bit confused because the original thinking was that
> vma_mark_detached() would drop the last refcnt and if it's 0 we would
> free the vma right there. If that's still what we want to do then I
> think the above sequence should look like this:

Right; sorry about that. So my initial objection to that extra sync was
based on the reasons presented -- but having had to look at the unmap
path again (my mm-foo is somewhat rusty, I've not done much the past few
years) I realized that keeping a VMA alive beyond unmapping PTEs is just
plain daft.

So yes, back to your original semantics, but cleaned up to not need that
extra sync point -- instead relying on the natural placement of
vma_start_write() after unhooking from the mm. And not for reasons of
the race, but for reasons of integrity -- VMA without PTEs is asking for
more trouble.