linux-kernel - Re: [PATCH 2/4] mm: perform VMA allocation, freeing, duplication in mm

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Message-ID: <7b176eaa-3137-42b9-9764-ca4b2b5f6f6b@lucifer.local>
Date: Fri, 25 Apr 2025 11:09:13 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Pedro Falcato <pfalcato@...e.de>, Kees Cook <kees@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] mm: perform VMA allocation, freeing, duplication in
 mm

On Thu, Apr 24, 2025 at 06:22:30PM -0700, Suren Baghdasaryan wrote:
> On Thu, Apr 24, 2025 at 2:22 PM David Hildenbrand <david@...hat.com> wrote:
> >
> > On 24.04.25 23:15, Lorenzo Stoakes wrote:
> > > Right now these are performed in kernel/fork.c which is odd and a violation
> > > of separation of concerns, as well as preventing us from integrating this
> > > and related logic into userland VMA testing going forward, and perhaps more
> > > importantly - enabling us to, in a subsequent commit, make VMA
> > > allocation/freeing a purely internal mm operation.
> > >
> > > There is a fly in the ointment - nommu - mmap.c is not compiled if
> > > CONFIG_MMU is not set, and there is no sensible place to put these outside
> > > of that, so we are put in the position of having to duplication some logic
>
> s/to duplication/to duplicate

Ack will fix!

>
> > > here.
> > >
> > > This isn't ideal, but since nommu is a niche use-case, already duplicates a
> > > great deal of mmu logic by its nature and we can eliminate code that is not
> > > applicable to nommu, it seems a worthwhile trade-off.
> > >
> > > The intent is to move all this logic to vma.c in a subsequent commit,
> > > rendering VMA allocation, freeing and duplication mm-internal-only and
> > > userland testable.
> >
> > I'm pretty sure you tried it, but what's the big blocker to have patch
> > #3 first, so we can avoid the temporary move of the code to mmap.c ?
>
> Completely agree with David.

Ack! Yes this was a little bit of a silly one :P

> I peeked into 4/4 and it seems you want to keep vma.c completely
> CONFIG_MMU-centric. I know we treat NOMMU as an unwanted child but
> IMHO it would be much cleaner to move these functions into vma.c from
> the beginning and have an #ifdef CONFIG_MMU there like this:
>
> mm/vma.c

This isn't really workable, as the _entire file_ basically contains
CONFIG_MMU-specific stuff. so it'd be one huge #ifdef CONFIG_MMU block with
one small #else block. It'd also be asking for bugs and issues in nommu.

I think doing it this way fits the patterns we have established for
nommu/mmap separation, and I would say nommu is enough of a niche edge case
for us to really not want to have to go to great lengths to find ways of
sharing code.

I am quite concerned about us having to consider it and deal with issues
around it so often, so want to try to avoid that as much as we can,
ideally.

>
> /* Functions identical for MMU/NOMMU */
> struct vm_area_struct *vm_area_alloc(struct mm_struct *mm) {...}
> void __init vma_state_init(void) {...}
>
> #ifdef CONFIG_MMU
> static void vm_area_init_from(const struct vm_area_struct *src,
>                              struct vm_area_struct *dest) {...}
> struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig) {...}
> void vm_area_free(struct vm_area_struct *vma) {...}
> #else /* CONFIG_MMU */
> static void vm_area_init_from(const struct vm_area_struct *src,
>                              struct vm_area_struct *dest) {...}
> struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig) {...}
> void vm_area_free(struct vm_area_struct *vma) {...}
> #endif /* CONFIG_MMU */
>
>
>
>
>
> >
> > --
> > Cheers,
> >
> > David / dhildenb
> >