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

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Message-ID: <20241218161343.GF2354@noisy.programming.kicks-ass.net>
Date: Wed, 18 Dec 2024 17:13:43 +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 Wed, Dec 18, 2024 at 10:37:24AM -0500, Liam R. Howlett wrote:

> This is also correct.  The maple tree is a b-tree variant that has
> internal nodes.

Right, I remembered that much :-)

> > Also, I think vma_start_write() in that gather look is too early, you're
> > not actually going to change the VMA yet -- with obvious exception of
> > the split cases.
> 
> The split needs to start the write on the vma to avoid anyone reading it
> while it's being altered.

__split_vma() does vma_start_write() itself, so that should be good
already.

> > That too should probably come after you've passes all the fail/unwind
> > spots.
> 
> Do you mean the split? 

No, I means the detach muck :-)

> I'd like to move the split later as well..
> tracking that is a pain and may need an extra vma for when one vma is
> split twice before removing the middle part.
> 
> Actually, I think we need to allocate two (or at least one) vmas in this
> case and just pass one through to unmap (written only to the mas_detach
> tree?).  It would be nice to find a way to NOT need to do that even.. I
> had tried to use a vma on the stack years ago, which didn't work out.

Urgh yeah, vma on stack sounds like utter pain :-)