linux-kernel - Re: [PATCH] mm/rmap: Add anon

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Message-ID: <CAG48ez1=rd5W5SBAyyqu-=50AsRGg8pdYW5BZ4SFhHqFYOHbyg@mail.gmail.com>
Date: Fri, 25 Jul 2025 13:24:55 +0200
From: Jann Horn <jannh@...gle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	Rik van Riel <riel@...riel.com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>, 
	Vlastimil Babka <vbabka@...e.cz>, Harry Yoo <harry.yoo@...cle.com>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/rmap: Add anon_vma lifetime debug check

On Fri, Jul 25, 2025 at 1:12 PM David Hildenbrand <david@...hat.com> wrote:
> On 25.07.25 13:08, Jann Horn wrote:
> > On Thu, Jul 24, 2025 at 11:56 PM David Hildenbrand <david@...hat.com> wrote:
> >> 2) We have a folio here, so ... better
> >>
> >> if (folio_test_anon(folio) && !folio_test_ksm(folio)) {
> >>          ...
> >> }
> >
> > Hrm, okay. It kind of irks me to write it as two checks when really I
> > want to ask "is it this one specific type", but yeah, will change it.
>
> Well, ksm is a subtype of anon.

I mean... not really? At least ksm folios are not a subtype of normal
anon folios. Normal anon folios point to an anon_vma, a ksm folio
points to a ksm_stable_node instead, and you can't treat a
ksm_stable_node as an anon_vma.