linux-kernel - Re: [PATCH] docs/mm: expand vma doc to highlight pte freeing, non-vma traversal

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Message-ID: <CAG48ez22QsH5NcE8+-_ofA185j1AiBFZNsaik338pjNr8kC-gw@mail.gmail.com>
Date: Tue, 3 Jun 2025 20:36:13 +0200
From: Jann Horn <jannh@...gle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Suren Baghdasaryan <surenb@...gle.com>, 
	"Liam R . Howlett" <Liam.Howlett@...cle.com>, Vlastimil Babka <vbabka@...e.cz>, 
	Shakeel Butt <shakeel.butt@...ux.dev>, Jonathan Corbet <corbet@....net>, 
	Qi Zheng <zhengqi.arch@...edance.com>, linux-mm@...ck.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] docs/mm: expand vma doc to highlight pte freeing, non-vma traversal

On Tue, Jun 3, 2025 at 12:45 PM Lorenzo Stoakes
<lorenzo.stoakes@...cle.com> wrote:
> On Tue, Jun 03, 2025 at 12:25:36AM +0200, Jann Horn wrote:
> > The one way in which I think this is currently kinda yolo/broken is
> > that vmap_try_huge_pud() can end up freeing page tables via
> > pud_free_pmd_page(), while holding no MM locks AFAICS, so that could
> > race with the ptdump debug logic such that ptdump walks into freed
> > page tables?
>
> But those mappings would be kernel mappings? How could ptdump walk into
> those?

/sys/kernel/debug/page_tables/kernel dumps kernel page tables. And I
think /sys/kernel/debug/page_tables/current_kernel dumps page tables
for the entire address space including both userspace and kernel.

> > (I think we should take all the vma locks in that ptdump code and get
> > rid of this weird exception instead of documenting it.)
>
> We really need to be sure that there aren't some weird architectures doing
> weird things or circumstances where this is meaningful.
>
> I mean people went to great lengths to make this possible, I find it hard
> to believe there aren't some _weird_ real world use cases.

FWIW, looking through the git logs for the x86 version of it, it seems
to mainly be used by developers of arch-specific code trying to
debug/validate kernel behavior.