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

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Message-ID: <20241218161850.GG2354@noisy.programming.kicks-ass.net>
Date: Wed, 18 Dec 2024 17:18:50 +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,
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	oliver.sang@...el.com, mgorman@...hsingularity.net,
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	dhowells@...hat.com, hdanton@...a.com, hughd@...gle.com,
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	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,
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Subject: Re: [PATCH v6 10/16] mm: replace vm_lock and detached flag with a
 reference count

On Wed, Dec 18, 2024 at 07:50:34AM -0800, Suren Baghdasaryan wrote:

> I think vma_start_write() should be done inside
> vms_gather_munmap_vmas() for __mmap_prepare() to work correctly:
> 
> __mmap_prepare
>     vms_gather_munmap_vmas
>     vms_clean_up_area // clears PTEs
> ...
> __mmap_complete
>     vms_complete_munmap_vmas

I'm unsure what exactly you mean; __split_vma() will start_write on the
one that is broken up and the rest won't actually change until
vms_complete_munmap_vmas().

> If we do not write-lock the vmas inside vms_gather_munmap_vmas(), we
> will be clearing PTEs from under a discoverable vma.

You will not. vms_complete_munmap_vmas() will call remove_vma() to
remove PTEs IIRC, and if you do start_write() and detach() before
dropping mmap_lock_write, you should be good.

> There might be other places like this too but I think we can move
> vma_mark_detach() like you suggested without moving vma_start_write()
> and that should be enough.

I really don't see why you can't move vma_start_write() -- note that by
moving that after you've unhooked the vmas from the mm (which you have
by that point) you get the sync point you wanted.