linux-kernel - Re: [RFC PATCH 4/6] mm/memory: Add tree limit to free

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Message-ID: <6b94ec3b-5f03-4201-8e5b-bbf2aec242ff@lucifer.local>
Date: Thu, 4 Sep 2025 16:36:17 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        maple-tree@...ts.infradead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Vlastimil Babka <vbabka@...e.cz>,
        Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jann Horn <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>,
        Charan Teja Kalla <quic_charante@...cinc.com>,
        shikemeng@...weicloud.com, kasong@...cent.com, nphamcs@...il.com,
        bhe@...hat.com, baohua@...nel.org, chrisl@...nel.org,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [RFC PATCH 4/6] mm/memory: Add tree limit to free_pgtables()

On Thu, Sep 04, 2025 at 12:20:55PM +0200, David Hildenbrand wrote:
> On 03.09.25 22:19, Liam R. Howlett wrote:
> > * Lorenzo Stoakes <lorenzo.stoakes@...cle.com> [250819 15:14]:
> > > On Fri, Aug 15, 2025 at 03:10:29PM -0400, Liam R. Howlett wrote:
> > > > The ceiling and tree search limit need to be different arguments for the
> > > > future change in the failed fork attempt.
> > > >
> > > > No functional changes intended.
> > > >
> > > > Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> > >
> > > (Obv. in addition to comment about broken VMA tests :P)
> > >
> > > I guess intent is that if we discover any page tables beyond tree_max then
> > > we ought to just wipe them all out so, in effect, we don't consider
> > > mappings at or past tree_max to be valid?
> >
> > Actually... there are some archs that map outside the vma and they are
> > valid.. I think mips? and I think lower, but yeah.. it's needed.  This
> > is why prev->vm_end and next->vm_start are used as page table limits,
> > afaik.  This is a serious annoyance because it frequently adds walks
> > that are infrequently necessary to the vma tree.
>
> Hm, does that still exist?
>
> I recall something odd ... was it that gate area thingy (in_gate_area) we
> also have to handle in GUP code? The is x86/arm though, not mips.

Isn't gate area the VSYSCALL area so that's basically a kernel mapping address
that we allow userland to access?

>
> --
> Cheers
>
> David / dhildenb
>