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

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Message-ID: <CAJuCfpGafwBehugTB+D2M03zsbnvO_L8zZb1gzHYtfu5uYugvg@mail.gmail.com>
Date: Thu, 19 Dec 2024 08:08:09 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
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 Thu, Dec 19, 2024 at 12:53 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> 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.

Ack. Thanks for clarification!